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==Contents==
==Contents==
[[Book - Experimental Embryology (1909) 1|Chapter I Introductory]]
[[Book - Experimental Embryology (1909) 1|Chapter I Introductory]]


[[Book - Experimental Embryology (1909) 2|Chapter II Cell-Division And Growth]]
[[Book - Experimental Embryology (1909) 2|Chapter II Cell-Division And Growth]]
 
# Ce1l-division
1. Ce1l-division
# Growth
2. Growth


[[Book - Experimental Embryology (1909) 3|Chapter III External Factors]]
[[Book - Experimental Embryology (1909) 3|Chapter III External Factors]]
 
# Grravitation
1. Urrnvitation
# Mechanical agitation
 
# Electricity and magnetism
2. Mechanical agitation
# Light
 
# Heat
Electricity and magnetism
# Atmospheric pressure. The respiration of the embryo.
 
# Osmotic pressure. The role of water in growth
Light
# The chemical composition of the medium
 
# Summary
Heat
 
Atmospheric pressure. The respiration of the embryo.
 
Osmotic pressure. The role of water in growth
 
The chemical composition of the medium
 
Summary


[[Book - Experimental Embryology (1909) 4|Chapter IV Internal Factors]]
[[Book - Experimental Embryology (1909) 4|Chapter IV Internal Factors]]


(1) The initial structure of the germ as a cause of differentiation.
(1) The initial structure of the germ as a cause of differentiation.
# The modern form of the preformationist doctrine
# Amphibia
# Pisces
# Amphioxus
# Coe-lenterata
# Ecliinodcrmata
# Nemertinen
# Ctenophora
# Chaetopoda and Mollusca
# Ascidia
# General considerations and conclusions
# The part played by the spermatozoon in the determination of egg-strucure
# The part played by the nucleus in differentiation


1. The modern form of the prefurmationist doctrine
(2) The actions of the parts of the developing organism on one another
2. Amphibia
3. Pisces
4. Amphioxus
5. Coe-lenterata
6. Ecliinodcrmata
7. Nemertinen . . . . . . . . 204
8. (.‘tenopho1':i . . . . . . . . 208
9. Chaetopoda and Mollusca . . . . . . 213
10. Ascidia . . . . . . . . . 229
11. General consiileratious and conclusions . . . 240
12. The part. played by the spernmtozoon in the determination of egg-.<ztructm'e . . . . . 247
13. The part played by the nucleus in ilifl'e1'enti;iti0n . . 251
(2) The actions of the parts of the developing oiganism on
one another 271


[[Book - Experimental Embryology (1909) 5|Chapter V Driesch’s Theories Of Development - General Reflections And Conclusions]]
[[Book - Experimental Embryology (1909) 5|Chapter V Driesch’s Theories Of Development - General Reflections And Conclusions]]
Line 109: Line 97:


APPENDIX A
APPENDIX A
On the .’~y)1)l)lL'l2l'y of the egg, the symmetly of scglnentation, and the symmetry of the embryo in the Frog  
On the symmetry of the egg, the symmetry of segmentation, and the symmetry of the embryo in the Frog  
 


APPENDIX B
APPENDIX B


On the part played by the nucleus in (lifferenti:L’tion
On the part played by the nucleus in differentiation
 
I.\'m<;x or AUTl{0I{
 
Ixmzx or SUBJPJCTS
 
ADDENDA
 
==Chapter I Introductory==
 
THAT living creatures reproduce their kind is a fact which is familiar to us all, but it is the peculiar privilege and province of the embryologist to observe and to reflect upon that marvellous series of changes whereby, out of a germ which is comparatively structureless and unformed, a new organism is developed which is, within the limits of variation, like the parents that gave it birth.
 
Development is the production of specific form. 1<‘rom a particular kind of germ only a particular kind of individual will normally arise, though unusual conditions may lead to the formation of an abnormality or monstrosity. Thus, while the germ is the material basis, development is the mechanism of inheritance. The student of heredity seeks to express in terms which shall be as exact as possible, ultimately mathematically exact, the degree of similarity between the offspring on the one hand, and parents and more remote ancestors on the other. The embryologist has under his very eyes the process by which that similarity is brought about, and even when the resemblance shall have been stated with all possible precision, it will still remain for him to give an explanation of those changes whereby the inheritable peculiarities of the species are handed on from one generation to the next.
 
Used in the widest sense of the word, development includes not merely the formation of a new individual from a single cell, whether fertilized or not, but also the phenomena of budding and regeneration. In a narrower sense, however, the term is restricted to the first of these processes, and a corresponding distinction is made, however artificially, between Experimental Embryology and Experimental Morphology, when the subject is treated from a physiological point of view.
 
In development two factors are obviously involved. One is growth, or increase of volume, more correctly increase of mass; the other is differentiation, or increase of structure; and, in multicellular organisms, both these factors are accompanied by division of the nucleus and the cell.
 
Segmentation is the first sign, or almost the first sign, the developing ovum gives of its activity; and this cutting up of the egg-cell into parts, which marks the beginning, is also continued during the later stages of ontogeny, and goes on as long as the life of the organism endures.
 
Growth is especially characteristic of the embryonic and of the adolescent organism. It occurs at different rates in the different cells, and indeed the growth of a group of cells is in itself often an act of differentiation. Growth may depend upon the absorption of water or the assimilation of other substances, and this may lead simply to an increase in the size of internal cavities, as in the blastula of lrlc-hinoderms or the Mammalian blastocyst; to an increase in the volume of the living protoplasm ; or to the secretion of intracellular or intercellular substances, either organic (for example, the notochordal vacuoles, the matrix of cartilage and bone) or inorganic (the skeletal spicules of Eehinoderm larvae, Sponges, and Coelenterata). This increase of mass is not only conditioned by the presence of food in the form of substances found in the environment, but depends on such external circumstances as temperature, atmospheric and osmotic pressure, and so forth.
 
But while the embryo is dividing up its material—a material which is already to a certain extent heterogeneous, composed, for example, of protoplasm and deutoplasm or yolk——while it is increasing its mass, it is also undergoing a process of differentia tion; and, as even a superficial acquaintance with embryology.
 
will inform us, one of the most characteristic features of differentiation is that it occurs in a series of stages which follow upon one another in regular order and with increasing complexity. When segmentation has been accomplished—sometimes, indeed, during segmentation——certain sets of cells, the germ-layers, become separated from one another. Each germlayer contains the material for the formation of a definite set of  organs——the endoderm of a Vertebrate, for instance, contains the material for the alimentary tract and its derivatives-—gill-slits, lungs, liver, bladder, and the like ; the germ-layers are therefore not ultimate but elementary organs, and elementary organs of the first order. In the next stage these primary organs become subdivided into secondary organs——as the arehenteron of an Eehinoderm becomes portioned into gut and eoelom-sac, or the ectoderm of an Earthworm into epidermis, nervous system, and nephridia——and in subsequent stages these again become successively broken up into organs of the third and fourth orders and so on, until finally the ultimate organs or tissues are formed, each with special histological characters of its own. This end is, however, not necessarily reached by all the tissues at the same time. Indeed, it is no uncommon thing for certain of them to attain their final structure while the others are yet in a rudimentary condition; thus, in some Sponges the scleroblasts begin to secrete spicules in the larval period, nematocysts may be formed in the Planula of the Coelenterates, notochordal tissue is differentiated in the newly hatched tadpole of the Frog; and, speaking generally, larval characters are developed at a very early stage.
 
To this regular sequence of ontogenetie events Driesch has applied the term ‘rhythm ’, the rhythm of development. The organs of the body are, however, by no means all formed of single tissues~—-bone, epithelium, blood, and the rest——but are compounded, frequently of very many tissues, and this ‘ composition ’, to quote a term of Driesch’s again, is another of the obvious features of organogeny.
 
VVhile, therefore, in the last resort all diiterentiation is histological, that final result, the assumption by the cells of their definitive form, is only achieved after many changes have taken place in the position of the parts relatively to one another while the organs are being compounded, and so its specific shape conferred upon the whole body.
 
It is possible to find a few general expressions for the manifold changes that take place in the relative positions of the parts. Several years ago, in 1874, His compared the various layers of the chick embryo to elastic plates and tubes; out of these he suggested that some of the principal organs might be moulded by mere local inequalities of growth——the ventricles of the brain, for instance, the alimentary canal, the heart—-and he further succeeded in imitating the formation of these organs by folding, pinching, and cutting india-rubber tubes and plates in various ways. This analysis, however, deals only with the foldings of flat layers, and must be supplemented by a more exhaustive catalogue of the processes concerned in ontogeny, such as that more recently suggested by Davenport. Davenport resolves the changes in question into the movements of cells or cell aggregates, the latter being linear, superficial, or massive, and within the limits of these categories the phenomena are susceptible of further classification. The catalogue proceeds as follows :——
 
I. THE Movmtaxrs or SINGLE Cams.
 
1. Migration of nodal thickenings in a network of protoplasm: e. g. the migration of the ‘ cells ’ to the surface of the Arthropod ovum to form a blastoderm, the movements of vitellophags, and yolk-nuclei (Fig. 1).
 
 
 
FIG. 1. — Sections of the egg of Geophilus ferrrlyiuezls showing two stages in the formation of the blastoderiu: bl, blastoderm; dp, yolk pyramids; gr, groups of blastoderni cells on what will be the dorsal side ; L-, nuclei surrounded by masses of protoplasm. (After Sograff, from Korsehelt and Heider.)
 
 
2. Migration of free amoeboid bodies: e. g. the mesenchyme cells in the Eehinoderm gastrula, the lower layer cells of Elasmobranchs, the blastomeres amongst the yolk-cells in Triclads and Salps.
 
3. Aggregation of isolated cells.
 
a. Linear aggregates: e. g. the kidney of Lamellibranchs, the yolk-gland of Turbellaria, capillary blood-vessels.
 
6. Superficial aggregates: e. g. the blastoderm of Arthropods, the formation of the imaginal gut-epithelium in some Insects.
 
0. Massive aggregates : e. g. the gemmule of Sponges, the spleen of Vertebrates.
 
4. Attachment of isolated cells to another body: e. g. the union of muscles to the shell in Mollusca and Arthropoda, of tendon to bone in Vertebrates, the application of skeletal cells to the notochord.
 
5. Investment and penetration by isolated cells: e. g. the follicle cells between the blastomeres in Tnnicata, the muscles of the gut in va.rious animals, the septa of the corpus luteum, the formative cells of the vitreous body of the Vertebrate eye, the immigration of the nephric cells in the Earthworm.
 
6. '1‘ransportation of bodies by wandering cells: e. g. of the buds in Doliolidae.
 
7. Absorption by wandering cells: e. g. phagocytosis in Insect pupae a11d in the ta(lpole’s tail.
 
8. We may place here. the frequent alterations in the shapes of cells, which do not apparently involve growth: e. g. when {lat cells become columnar.
 
II. Tm»; Movi~:.\mNTs or (J1~:r.r. A(:eu1«;(:A'rEs. A. Linear Aggregates.
 
1. Growth in length: e. g. the growth of the roots and stems of plants, of the stolons and hydranths of Hydroids, the outgrowth of nerves, of the necks of unicellular glands, the growth of the blood-vessels from the area vaseulosa into the body of the Chick embryo, of blood-vessels towards a. parasite, the growth of mesoblastie and other germ-bands in Annelids, the back-growth of the Vertebrate segmental duct, and the like.
 
2. Splitting.
 
a. At the end, that is, branching: e.g'. of nerves, bloodvessels, kidney tubules, glands, tentacles.
 
3. Throughout the length: e.g. the segmental duct of Elasmobranehs, the truncus arteriosus of Mammalia.
 
3. Anastomoses: e. g. of the dorsal and ventral roots of the spinal nerves, of nerve plexuses, of capillaries, of bile capillaries, of the excretory tubules of Platyhelmia.
 
4«. Fusion with other org'a11s: e. g-. of a nerve with its endorgan, of the vasa eiferenti-.1. with inesonephrie tubules, of nephriclia with the coelom in Annelida.
 
B. Superficial aggregates. i. Increase of area. (1.. Growth of a sphere. 1. Equal in all dire(-tions: e.g. the blastula of ]*l(-hinoderms. 2. Unequal.
 
oz. Unequal in dilferent axes: e.g-. the conversion of a spherical blastula into an ellipsoid Planula in Coelenterata, or into an ellipsoid Sponge larva, or of the spherical into the ellipsoid blastocyst in l\Ia1mnalia.
 
,8. Unequal at dilferent poles: e. g. the formation of ovoid forms, such as 1’lannlae, the club-shaped glaild of .122/19/ziamzs, the auditory vesicle of Vertebrata.
 
6. Growth of a plane surface. 1. Equal in all directions: e.;,;'. the growtli of the blastodcrm over the yolk in Sauropsida, or Cephalopoda.
 
2. Unequal. a. When parts lying in one plane move out of that
 
plane: c. g. inva,t,;‘i11ations and evaginations of all descriptions.
 
(Fig. 2).
 
B. When parts—e. a row of eells——lying in one plane are moved in that plane: e. g. the germ-bands of C/cpsinc, by the growth of the epiblast (Fig. 3).
 
ii. Alterations of thickness. 1/. Increase: thickenings: e. g. the formation of the central nervous system in Teleostei, the formation of gonads from the I INTRODUCTORY 7
 
coelomic epithelium, the development of hair follicles, the trophoblast in the Mammalian placenta (Fig. 4).
 
 
 
Fm. 2.—-Three stages of an invagination or evagination. (After Korschelt and Ileider.)
 
 
FIG. 3.——Displacement of a row of cells in an epithelium. (After Kor- qQp§§ag!d9“’gQ‘éDO
 
schelt and Heider.) PI 4 F t _ tlef t, < .——— ours ages 1n 1 orma 1on
 
uf an epithelial thickening of many layers. (After Korsehelt and Heicler.)
 
6. Decrease: thinnings: e. g. in the roof of the thalamencephalon a11d medulla, the outer layer of the lens, the trophoblast of the Mammalian blastocyst. 8 INTRODUCTORY I
 
iii. Interruptions of continuity. a. By the atrophy of part of a, layer: e. g. when the floor of the archenteron tog-ether with the underlying paraderm dis appears in Amniota. (Fig. 5). X . ° UODCIDDUUDDDUDDU °
 
3 « <2 vacuum
 
5 canon» «aaaaaan
 
Fig. 5.——Three stages in the development of am interruption of von~ tinnity pcrpemliculzmr to the surfztce of am epithelium. (l'erfomtion.) (After Korschelt and Heidcr.)
 
/1. By the detachment of a part: c. g. of the medullary plate from the eetoderm in Amp//ioxus (Fig-. 6), of the notochord from the roof of the archenteron in Urodela. and I’e/r0myzo7z.
 
 
 
 
FIG 6. - Scheme of the formation of the medullary canal in Amphi0.rIts. (After Korschclt and Heidcr.) I INTRODUCTORY 9
 
iv. Conerescence of layers. a. By their margins: e. g. the edges of the eetoderm over the medullary plate, the edges of the embryonic eetoderm inside
 
 
 
 
Fm. 7.- Fusion of two cell plates by their nmrgins. (After Korschclt and Heider.)
 
the serosa of Si/nmczzlzts, the embryonic plate with the trophohlast in some Mammals (Figs. 7, 9).
 
(2. By their surfa.c-es (Figs. 8, 9, 10) : e. g. when the stomodaeum or proctodaenm open into the gut, when the mcdullary
 
 
 
FIG. 8. — Fusion of two cell plates by their surfaces. (Aft-er Korsehelt and Heider.) folds meet, when the edges of the peritoneal groove close to form the canal of the oviduct in Amphihia and Amniota.
 
This concrescence is commonly followed by a communication of the cavities on opposite sides of the adherent layers, as when the stomodaeum opens into the gut, or the amnion-folds unite ; but not necessarily, as when the somatopleure fuses with the trophoblast, or the allantois with the somatopleure in Mammalia.
 
v. Splitting of a. layer into two: e. g. the inner wall of the pineal vesicle in Lacertilia (Fig. 11). 10
 
 
 
FIG 11. — Three stages in the development of an interruption of continuity parallel to the surface of an epithelium. (Delanlination) (After K01-schclt and Heider.)
 
 
0. Massive aggregates.
 
i. Changes in volume.
 
a. Unequal in different axes : c. g. when the spherical larva becomes cylindrical in Dicyemidae.
 
3. Unequal at diflerent points : e. g. the outgrowth of limbbuds of Vertebrates and other forms, of the buds of plants.
 
ii. Rearrangement of material.
 
a. Simple rear1‘angement of cells: e.g. in the formation of the concentric corpuscles of the thymus, in the development of kidney tubules in the metanephric. blastema of Amniotn, in the grouping of the cells to form ectoderm, gut and atrium in the Salps.
 
b. Development of an internal cavity: e.g. segmentation cavities, lumina of ducts and blood-vessels, of the coelom and many generative organs.
 
c. Dispersion ol’ the elements of an aggregate: e. g. in gcmmule formation in certain Sponges, in unipolar immigration in some Sponges and some (‘oelcntcrates, in the liberation of the germ—celis.
 
iii. Division of masses.
 
a. By constriction : c. g. the segmentation of the mesodcrm and neural crest.
 
b. By splitting: e. the nervous system from the ectodcrm in Teleostei and many Invertebrates, the notochord from the roof of the {l.1'('l1(3lltCl'011.
 
iv. Fusion of masses : e. g. of originally separate nerve ganglia (Vertebrates, Arthropods, Annelids), of myotomes, of somites in Artln'opods.
 
v. Attachment of one mass to another: e. g. of selerotome to notoehord.
 
It will be seen that this ;'¢7.s-/rzzr/‘"3 17 1C principal kinds of movement executed by the dev 11111:) ‘arts extends Ilis’s principle of the local inequality of growth from flat layers to linear and massive aggregates and at the same time includes the movements of isolated cells. Davenport, however, is not content merely to give a simple classification ol the phenomena; he goes further, and endeavours to express them in terms of responses to stimuli, an idea due in the first instance to Ilerbst. 12 INTRODUCTORY I
 
Thus he suggests that the migrations of vitellophags and mesenchyme cells, the thickenings, thinnings, and perforations of flat layers, the rearrangements of cells in a massive aggregate, their dispersion, the constriction, and splitting and fusion may be regarded as tactic responses, the growth in various ways of linear aggregates, the eoncrescence of layers and masses as so many tropic responses to stimuli which may be positive or negative and exerted by other organs or by agents in the world outside.
 
Now it is clear that the analyses both of His and Davenport aim at something more than a mere description of ontogenctic events, for a serious attempt is here made to give a causal, if you will a mechanical, explanation of those events, and the subject thereby raised at once from the level of mere morphology or morphography to a loftier, aetiological point of view.
 
There are, indeed, two methods by which embryology, like any other branch of zoology, may be investigated. One is purely descriptive, anatomical, morphological. By this method, truly, great results have been achieved. The life—l1i_stories of members of all the most important groups of the animal kingdom have been worked out, and the science of Comparative Embryology has been built up. Nor has an explanation of the process been lacking. For ontogeny is, the fundamental Biogenetic Law assures us, a recapitulation of and therefore explicable in terms of phylogeny; and since on this principle the individual repeats in its development the ancestry of its race, embryology affords a means of tracing out the relationships of the organism and establishing the homologies of its parts.
 
Unfortunately a more intimate acquaintance with the facts has made it abundantly clear that development is no mere repetition of the aneert ‘é~\(_‘ "e1,tliat the organism has manifold ways of attaining its sin k i" ‘.',(\at those resemblances in early stages which were held to .,.:sm. bthe most triumphant vindication of the Biogenetic Law bear no constant relation to the similarities of adult organization, that the attempt to find in development an absolute criterion of homology is vain.
 
The facts thus remain unexplained, as in truth it was only to be supposed that they would. A method, however comparative, which relies on mere observation, and is content to wait for Nature's own experiments, cannot hope to arrive at sound inductions, or to establish general laws of causation.
 
There is, however, another \vay. Development, the production of form, may be regarded as one of the activities, one of the functions of the organism, to be investigated. like any other function by the ordinary physiological method of experiment; and the ideal of the experimental or physiological embryologist is to give a complete causal account, whether the causes are external or internal, of each stage, and so of the whole series of ontogenctie changes, his weapon, to borrow Roux’s splendid phrase, ‘die Geistesanatomie, das analytische eausale Denken.’ 1
 
This effort is, of course, no modern one. Speculation into the nature and essence of development begins, indeed, with the Greeks, and theories of fertilization and development are to be found in the writings of Aristotle? In fertilization the male element, which, according to Aristotle, provides the formal and efficient causes in providing the necessary perceptive soul, acts upon the mere matter, endowed only with a nutritive soul, which is given by the female, in the same sort of way, to use his own illustration, as rennet coagulates milk. In the germ thus formed the parts of the embryo, which can only be said to pre-exist potentially, arise not simultaneously but in gradual succession, first the heart, then the blood, the veins from the heart and the various organs about the veins by a process of condensation and coagulation, the anterior parts of the body being built up first.
 
This Aristotelian doctrine appears to have persisted through the Middle Ages ; it reappears in the seventeenth century in the pages of Hieronymus Fabricius ab Aquapendentc and his pupil VVilliam Harvey in essentially the same form, although both authors diifer from Aristotle in certain matters of observational detail. Thus l*‘abricius3 states that ‘ ope generatricis facultatis pulli partes, quae prius non erant, produci atque ita ovum in pulli corpus migrate’, while Harvey“ gives to development as thus conceived of the name of ‘Epigenesin sivc partium superadditionem’, though he believes that in some cases (Insects) the
 
1 Roux, 1885. __ '2 Aristotle, De Gem, i. 20. 18; ii. 4. 43; 5. 2, 3, 10; 6. DeAM., 11.4. 2, 15 ; 5; 6. 3 Fabricius, 1. c., p. 22. ‘ Harvey, l. c., Ex. 44.
 
process is one of ‘metamorphosis’ or the simultaneous origin of all parts. In generation properly so called, however——in the development of the Chick, for exa1nple——tl1e process ‘a parte aliqua, tanquam ab origine, incipit; eiusque ope reliqua membra adseiscuntur: atque haec per epigenesin fieri dieimus: sensim nempe, partem post partem’, and this ‘pars prima genitalis’ Harvey held, in opposition to Aristotle, to be the blood.‘
 
But in spite of the exact observation and brilliant exposition of his followers, the teaching of Aristotle was destined to be overshadowed and eclipsed, tempoiarily at least, by a new l1ypo— thesis which, appearing first towards the end of the seventeenth century, swept the schools and universities, and dominated biolog-ieal speculation for a hundred years.
 
This was the theory of Evolution or Preformation. According to it the future animal or plant is already present in miniature in the. germ with all its parts complete, invisible or hardly visible it may be, but still there, and not merely ‘ potenti-ft’; and in development there is no such thing as ‘generation’, but only growth, whereby that which was before impalpable and invisible becomes tangible and manifest to our eyes. A further and logical outcome of the hypothesis was the doctrine of ‘emboitemcnt’, enthusiastically described by Bonnet as ‘une (les plus belles vietoires que l’entendement pur ait remporté sur les sens ’.3 The organism present already in the germ, with all its parts complete, possesses of necessity the germs of the next generation, and so on in indefinite though not in infinite regress, for as Bonnet is careful to tell us, ‘ Il ne faut pas supposer un emboitcment in l’infini, ce qui scroit absurde. La divisibilité de la matiére 51 l’infini par laquelle on prétendroit soutenir eet emboitement est une vérité géométrique et une erreur physiqne.’3 Swammerdam solved the difliculty in another way. All the’ germs of the human race must have been present. in the bodies of our first parents, and ‘exhaustis his ovis humani generis finem adesse ’.“
 
The theory became widely held. First put forward by Marcello
 
‘ Harvey, l. e., EX. 50.
 
2 Bonnet, Cont. de la nm‘., 71"“ partie, c. ix ; (1a'uLv'cs, vol. iv, p. 270. 3 Bonnet, Consid. sur les rorps 0131., c. viii; (lfiuvres, vol. iii, 1). 74.
 
‘ Swammerdam, 1679, pp. 21, 22.
 
Malpighi in the memoir, ‘De formatione pulli in ovo,’ which he presented to the Royal Society of London in 1673, it was not only adopted by biologists of prestige, by Swammerdam, Haller, who in his early days had been an advocate of Epigenesis, dc Buffon, and Bonnet, but secured the adherence of philosophers of such eminence as Malebranche and Leibniz.
 
In some cases it was accepted as a result of observation. Thus Malpighi,‘ in the treatise referred to, asserted that he had himself observed the chick in the unineubated egg, ‘ inelusum foetum animadvertebam, cuius caput cum appensae carinae staminibns patenter emergebat,’ while de Buifonz expresses himself even more categorically. ‘ J ’ai ouvert,’ he says, ‘ une grande quantité d’eeufs a difiérens temps, avant et apres Pineubation, et je me suis eonvaincu par mes yeux que le poulet existe en entier dans le milieu de la eicatricule a11 moment qu’il sorte (lu corps de la poule.’ To others, however, it was rather a matter of theoretical necessity. Ilaller explains his conversion from the contrary opinion by asking‘ the very pertinent question, ‘(‘ur vis ea essentialis quae sit unica tam divorsas in animali partes semper eodem loco, semper ad eun(lem arehetypum struit ; si materies inorganica mutabilis et ad omnem figuram rccipiendam apta est ? Cur absque ullo errore ex gallinac mista materie ea vis semper pullum, ex pavo11e pavonem fabricatur? ’
 
‘Nil nisi vis dilatans et progrediens reeipitur. Ab ea nihil sperarem nisi vasorum rete tamdiu continue amplius futurum quamdiu vis expandens resistentiae superandae par est. Cur loeo eius retis cor, eaput, eerebrum, ren struuntur? Cur in singulo animali suus ordo partium? Ad eas quaestiones nulla. datur responsio,’ a charge which is, of course, perfectly just.“
 
Bonnet’s argument is different. The heart of the chick, he points out, is already present in the egg‘; and since anatomy teaches that all the parts of an animal ‘doivent avoir toujours coexisté ensemble’, preformation follows as a matter of course.“
 
The belief in preformation continued paramount till towards the end of the eighteenth ceutur , nor was it till the publication in 1774 of Caspar Friedrich Wolff's T/Ieoria Gczze/'atz'om'.9 that the evolutionists were aroused from their dogmatic slumbers. Putting speculation on one side, Wolff returned to the method of Harvey, Fabricius, and may we not say also of Aristotle, the method of exact observation. He demdnstrated the presence in the unincubated egg not of a complete organism, but of ‘globules’; ‘ partes enim eonstitutivae, ex quibus omnes corporis animalis partes in primis initiis componuntur, sunt globuli," and described the epigenetic formation of the heart and blood-vessels, the central nervous system, the limbs and the ‘ Wolffian’ bodies from these primary elements.
 
‘ Malpighi, 1. e., p. 4. 2 do Buffon, 1. c., p. 351. 3 Haller, 1778, VIII. i. 29, p. 121. _ ‘ Bonnet, Cont. de la nat., 7""? partie, c. ix; (lc'm‘)‘fls, vol. 1v, 1). 261.
 
 
Development thus consists of the gradual production and organization of parts; ‘embryonis partes sensim produci, mea observata suadent,’ 2 and again, ‘ suppeditari prius partem, deinde cam organisari iutelligitur ’.3
 
The ground was thus taken from beneath the feet of the preformationists, and Epigenesis restored to its former place of honour as the fundamental expression of developmental fact.
 
Tacitly accepted by all the great embryologists of the nineteenth century—~Pander, von Baer, Reichert, Bischoif, Remak, Kolliker, Kowalewsky, IIaeckel——the epigenetie idea continued to control the progress of research. These were men who set themselves to describe the sequence of changes that the embryo passes through with all possible accuracy, and over as wide a range as might be of animal form. 'l‘hey made Comparative Embryology. On the facts that they discovered new light was shed by the doctrine of descent with m0(lilicati0n, or evolution in the wider sense of the word. Von Baer had pointed out that in any group of animals the embryos were more like one another than were the adult organisms, and this now became easily translated by Haeekel into the idea that the form which is in every g-roup——ultimately in all groups—thc common startingpoint of individual development is representative of the common ancestor of the race. Ontogeny was thus not merely expressed but explained in phylogenetic terms.
 
Now that, as we have already seen, the proposed explanation has very‘ largely broken down, Epigenesis, taken by itself, remains, not a theory in terms of cause and effect, but a mere description of what occurs, and it is the crying need for such a theory that has given birth to modern experimental embryology.
 
‘ Wolf, 1. c., Praemonenda, lxviii. 2 Id. ib. lxxiii. 3 Id. ib., De Gen. An., § 240.
 
 
 
The new era opens with the publication by Wilhelm His in 1874, just a hundred years after the appearance of the '1’/eeoria G'e2wratiom's, of a remarkable series of essays entitled Uusere K5f])87f07'7)l marl Jae 1;/1_'/siologisc/10 Problem 2'/zrer Eutste/nm_q. In these essays His, who was already in revolt against the ‘ Biogenctie Law’, not only sought to give a mechanical explanation of differentiation, but also laid down his famous ‘Prinzip der organbildenden Keimbezirke.’ According to this principle of ‘germinal localization’, every spot in the blastoderm corresponds to some future organ : ‘ (las Material zur Anlage ist schon in der ebencn Keimseheibe vorhanden, abcr morphologisch nieht abgcglie(lert, und somit als solchesnicht ohne Weiteres erkennbar.’ 1 Conversely, every organ is represented by some region in the blastoderm, and ‘ wenn wir consequent sein wollen’ in the fertilized, or even unfertilized, egg. In other words, although the parts of the embryo cannot be said to be preformed in the germ, the materials for those parts are already there, prelocalized, arranged roughly, at least, as the parts themselves will be later on. In this material unequal growth produces the form of the parts, and so of the whole body. Whether there is a strict causal connexion between each material rudiment and the organ which arises from it, whet-her these rudiments could be interchanged without prejudice to the normality of subsequent development, is a question which is not touched upon by His. It was reserved for another anatomist, Wilhelm Roux, to raise what in IIis’s hands had been merely a principle to the rank of a theory, the ‘ Mosaiktheoric ’, or theory of self-diiferentiation.
 
1 His, 1. c., §ii.
 
For Roux, no doubt, the ‘Mosaik-theoric’ was in part the outcome of the theoretical necessity of explaining the specific nature of development ; but it rests also upon a basis of observation and experiment. The coincidence in a majority of Frogs’ eggs of the first furrow with the sagittal plane, the production of local defects in the embryo by local injuries to the egg, th occurrence of certain natural monsters (Hemit/leria anteriora, for example) in which one half of the body is normally developed, the other entirely suppressed, and the experimental demonstration of the formation of a half-embryo from one of the first two blastomercs of the Frog’ s egg when its fellow had been killed, all led Roux to regard the development of the whole and of each part as essentially a process of self-differentiation, a process, that is to say, of which the causes reside wholly within the fertilized ovum and within each part as it is formed, though allowance was made for the possible formative influence of the parts on one another in later stages. External conditions, though they may be necessary in the same sense as they are generally necessary to the maintena.nce of life, are yet of no importance for diiferentiation regarded as a specific activity of the organism.
 
 
In the meantime, an experiment of Pfluger’s had apparently shown that, however obviously each part of the egg-cell might be related to the production of a particular organ, the relation was no necessary one, but that, on the contrary, the parts were all equivalent and the ovum ‘isotropic’. Pfliigcr demonstrated that in a Frog-’s egg which had been prevented from assuming its normal position with the axis vertical, the planes of the segmentation furrows bore no constant relation to the original egg-axis, that is to say to the structure of the egg, though they exhibited the same relation to the vertical as when de— veloping in the normal position. Further, in such forcibly upturned eggs the plane which included the original egg-axis and the present vertical axis became the median plane of the embryo, whose axes were disposed with regard to the vertical as in normal cases. Any part of the egg, therefore, might give rise to any part of the embryo, according to the extent to which and the direction in which the egg-axis had been diverted out of its original vertical position, and hence the egg-substance was ‘isotropic’ ; the planes of segmentation and the embryonic axes being determined by gravity. In fact, Pfliiger went so far as to say that an egg only becomes what it does become because it is always placed under the same external conditions.
 
 
Nor was this conception of the isotropy of the ovum invalidated by Born’s proof that in these eggs there is a redistribution of yolk and protoplasm owing to the sinking of the former to the lower, the rising of the latter to the upper side of the egg. For though the egg thus comes to acquire a secondary structure about an axis which is vertical, still the arrangement of the parts of the supposed rudiments of the organs must have been disturbed. Yet from such ova normal tadpoles are developed.
 
 
It became necessary, therefore, to locate the self-differentiating substance, the ‘idioplasma’, mainly, at any rate, in the nucleus; and this idioplasma was imagined as composed of dissimilar determinant units, each representative of some part or character of the organism and arranged according to a plan or architecture which corresponds in some way with the architecture in the embryo of the organs represented. All that is necessary, then, and all that happens, at least in the early stages of development, is the gradual sundering of these units from one another by successive qualitative divisions of the nucleus and their distribution to the cytoplasm, where each determines the assumption by the cell to which it is allocated of that character which it represents.
 
Roux’s ‘Mosaik—theoric’ (Roux, 1903) and Weismann’s very similar but more elaborate hypothesis of the constitution and behaviour of the germ-plasm both frankly involve the belief that every separately inheritable quality of the body has its own representative in the germ, with the difference, however, that this preformation, extended by Weismann to the adult characters, is limited by Roux to those of the embryo. The renewed inquiry into the nature and essence of development has thus simply resulted in the resuscitation of the eighteenth-century doctrine of evolution, though in a far more subtle form. Once again we find ourselves face to face with the old alternative, Preformation or Epigenesis ,- and it is to the desire of solving this problem that a very considerable proportion of modern experimental research is attributable. Though much of this has been directed against and been destructive of the ‘ Mosaik-theorie’, which as far as the nucleus is concerned has now been abandoned by Roux himself} renewed investigation has proved the existence in many cases of definite and necessary organ-forming substances in the cytoplasm, while the necessity for finding a causal explanation of what is obviously in some sense a predetermined process, without presupposing the preformation in the germ of morphological units representing every possible iuheritable character, has issued in Herbst’s and Driesclfs conception of the events of ontogeny as so many responses to stimuli exerted by the developing parts on one another. At the same time the need for inquiry into the external conditions and the part they may play in growth and differentiation has not been forgotten.
 
Thus though it would be vain to pretend that the ideal of a complete causal explanation has yet been realized, still some material has been gathered for an answer to each of the two main questions: what are the internal, what the external conditions that determine the course of development? These questions we shall discuss in the following pages. It will then only remain to inquire whether a causal explanation—-in the accepted sense of the phrase—a mechanics of ontogeny which resolves the single occurrences first into general physiological laws, and these in the last resort into the generalizations of physics and chemistry, can ever afford a theory which may be said to be complete either from a scientific or from a philosophical point of view. Should the mechanical explanation prove to be scientifically insufficient, it may be necessary, with Driesch and the neo-vitalists, to invoke a consciousness of the end to be realized to guide and govern the merely material elements; but even were this not so it would still be incumbent upon us to consider whether the end itself—not the consciousness of it—is not the final, and yet none the less the principal determining cause of the whole process.
 
===Literature===
 
ARISTOTLE. Dc gcneratione animalium, ed. Bekkcr, Oxford, 1837. De partibus animaliuin, ed. Bekker, Oxford, 1837. De anima, ed. Trendelenburg, Berlin, 1877.
 
C. BONNET. (Euvres, Neuchatel, 1781. Contemplation do In. nature, vii“‘° partie; Considerations sur les corps organises; Mémoire sur les gerlnes; Palingénésie philosophique.
 
G. BORN. Ueber den Einfluss der Schwere ant‘ das Froschei, Arch. mi/rr. Anaf. xxiv, 1885. I INTRODUCTORY 2 1
 
L. DE I1UFI~‘()N. Histoirc nuturelle, génc’-mle et. pzl1'ti('111ii:1‘e, vol. ii, 1’zu'is, 1749.
 
C. B. DAV!-:N1*oR'1‘. Studies‘ in morphogenesis: iv. A prelimimtry catalogue of the processes concerned in ontogeny, Bull. 1Iarmr«l Mus. xxvii, 1896.
 
H. DRIESOII. Anzrlytische Theorie der orgzmischen Entwicklung, Leipzig, 1894.
 
IL FABRICIUS AB AQUAPENDENTE. De formatione ovi et pulli. Opera. omnia, Lipsiae, 1687.
 
A. HALLER. Hernmnni Boerhanve pmelectiones ncndemicae: edidit at notas addidit Albertus Haller, v, Gottingne, 1744. Prinme lineae physiologiae, Gottingae, 1747. Elementa. physiologiae corporis humani, Lausannae, 1778.
 
W. HARVEY. Exercitationes dc genemtione :mima.1ium, London, 1651.
 
C. Ilmmsr. Ueber die Bedeutung dcr Reizphysiologie fiir die emisale A11f'f'zLss11ng' V011 Vorga"mgeu in der thierisclien Ontogenese, Biol. (‘mil-albl. xiv, xv, 1894, 1895.
 
\V. HIS. Unsere Korperforin nnd das physiologisclle Problem ihrer Entstehung, Leipzig, 1874.
 
G. W. LEIBNIZ. (Euvres philosophiques, ed. by 1’. Janet, Paris, 1900. Systeine nouvenu de 1:1. natlire, 1695. Principes de ]:1. nature at de 1:1. gr-free, 1714. Essnis do Théodicée. Préfmze, 1710.
 
N. MALEBRANCHE. Recherche dc la. vérité, Si1non’s edition, Paris, 1846.
 
M. MALPIGIII. De formatione pulli in ovo, London, 1673.
 
E. PFLUGER. Ueber den Einfluss der Schwerkmft uuf die Theilung der Zellen und auf die Entwieklung dos Embryo, Pflc7,«/m".s- An-h. xxxi, xxxii, 1883.
 
W. ROUX. Ein1eit.11n;;' zu den ‘ Bcitrfigen zur Entwicklungsmechunik des Embryo ’, Zeitsc-hr. Biol. xxi, 1885 ; also Ge.»-. Abh. 13, Leipzig, 1895.
 
W. Roux. Ueber die kiinstliehe Hervorbringung ‘ha.1ber’ Embryonen, V4'rrIum‘s Arch., 1888 ; also Gas-. Abh. 22.
 
W. Roux. Ueber Mosuikarbeit mid neuere Entwieklungslnypothesen, Anat. Ilrjffe, 1893 ; also G'(‘.s'. Abh. 27.
 
VV. Roux. Einleitung, Am-I1. Eut. Jllwclz. i, 1895.
 
VV. ROUX. Fiir unser 1’rogramn1 und seine Verwirkliehung, Arrlz. Ent. Mech. v, 1897.
 
VV. RUUX. Ueber die Ursuchen der Bestimmun,g' der H:lll])1.l'iC]|tnngen des Elnbryo im Frosclici, Anat. Anz. xxiii, 1903.
 
\V. ROUX. Vortri'Lgc und Aufsdtzc fiber Entwvicklungsliieelmnik (ler O1'ga.nismen, Leipzig, 1905.
 
J. SWAMMERDAM. Mimculum na.tura.c sive uteri muliebris fabrica, Lugdunulu Buta.vorun1, 1679. Histoire génémle des Insectes, Utrecht, 1682.
 
C. F. WOLFF. 'I‘heo1-in generationis, Hnlle a. d. Szmle, 1774.
 
==Chapter II Cell-Division and Growth==
 
===1. Cell-Division===
 
IN a future chapter we shall see that there is no necessary connexion between segmentation and differentiation. Nevertheless, since ccl1~division is the first sign, or almost the first sign, that a developing organism gives of its activity; since, moreover, cell-division accompanies the later processes of growth and differentiation, we may briefly discuss what is known of those factors which determine the direction of division in general, and in particular the pattern of segmentation.
 
We shall first presume that segmenting ova may be grouped under several distinct types, as follows :—
 
1. Tim radial I_1//2e. Here the first division is meridional, the second meridional and at right angles to the first, the third equatorial—or more often latitudinal—-and at right angles to both the preceding, the fourth meridional and at forty-five degrees to the first two, the fifth latitudinal. What is characteristic above all of this type is, first, that four surfaces of contact between cells meet in one line; for example, the four surfaees between the first four blastomeres meet in the egg-axis, while each pair of animal cells lies exactly over each pair of vegetative cells after the third division; and secondly, the blastomeres are radially arranged about the axis. This type has been observed in Sponges (Syconl (Sehulze)), in Coelenterates, in Crinoids, Holothurians, and Echinoids (Fig. 12) amongst the Echinoderms, in Eetoproctous Polyzoa, in A111];/ziowz/.e and the
 
Vertebrates, and in some C1'11stacea—Cc/or/zilzts ((:‘rrobben),’
 
Luc-g'f'er (Brooks), Cyclops (Hiickcr), L’/‘awe/u'p11.v (Braucr) and some Cirrhipedes. Certain of these cases present special
 
peculiarities. In Echinoids micromeres are formed at the vegetative pole
 
by the division of the fourth phase (Fig. 12,
 
' In 831001: the third cleavage is meridional, the fourth latitudinal. FIG. 12.-Normal development of the sea-urcliin SI;'ong3/Ionenlrolus ”l’i4Il(.\‘. (After Boveri, 1901.)
 
The animal pole is uppermost in all cases, and in the first two figures the jelly with the canal (micropyle) is shown.
 
a, primary oocytc, the pigment is uniformly peripheral.
 
b, ovum after extrusion of polar bodies. The pigment now forms a subequatorial band. The nucleus is ex-axial.
 
c, d, first division (meridional).
 
e, 8 cells, the pigment almost wholly in the vegetative blastomeres.
 
f, formation of mesomeres (animal cells) by meridional division: the vegetative cells have divided into macromeres and micromeres.
 
_r/, blastula. h, mesenehyme hlastula.
 
1', j, k, imagination of the pigmented cells to form the archenteron of the gastrula. In j the primary mesenchyme is separated into two groups, in each of which, in Ir, a spicule has been secreted. In is the secondary, pigmented mesenchyme is being budded off from the inner end of the archenteron.
 
 
In Asteroids and Ophiuroids the division is at first tetrahedral, and to be classed, therefore, with those of the following type; after the second furrow, however, the blastomeres are rearranged, and division theneeforward is radial.
 
In Vertebiata segmentation is altered in megaleeithal eggs by the amount of yolk present. It becomes meroblastic; still the radial type is preserved, though the sequence of the furrows is often altered, the third, for instance, being frequently meridional, and the fourth latitudinal. Amongst the Ascidians Pg/rosoma has a large-yolked, telolecithal, and radially segmenting egg. In the Placental Mammals the first two (livisions may conform to this type; but segmentation soon becomes irregular. The accumulation of yolk in the Arthropod egg has resulted in a totally different type of meroblastic segmentation. The yolk is here uniformly distributed about the central protoplasm. In the latter is placed the segmentation nucleus, and this central mass divides into a number of cells, which subsequently migrate to the surface and form a blastoderm ; the egg is then centrolecithal (Fig. 1). The stages of the development of this modification may be seen in the Crustaeea. In certain forms-——thosc alluded to above (with the exception of the (‘irrhipedes)—— division is holoblastic and radial. In (fam/mru.\', Branc/zipux (Brauer), Pellogastcr (Smith) segmentation is at_first total, but the inner yolk-containing ends of the cells subsequently fuse. In C’1'a7z.r/‘on, ]l[0i/ca, I)aj)/mella, ])a[//erzia, Ore/wstia segmentation is superficial. In Isopods and in Deeapods segmentation is internal. In all cases the result in the end is the same, a peripheral blastoderm, a central yolk. But the blastoderm is not always, though it is often, formed simultaneously over the whole surface. There are cases in which it appears first on the ventral side, and by what may be described as a still more precocious formation of the blastoderm, segmentation may begin at this, the future ventral, point, as in Jlfysis and 0m'scu.9. In these cases the egg is teloleeithal.
 
In the Insects, Arachnids, Myriapods, and Perz';)atus nomezealandiae, the segmentation is meroblastic and the egg comes to be centrolceithal. In Peripatzw capevms and in some other species it would appear that the yolk has been secondarily lost. II. I CELL-DIVISION 25
 
2. The‘ second type is the so-called spiral form of cleavage (Fig. 13). This is especially characteristic of the eggs of Polyclads, Nemert-ines, Molluscs (except Cephalopods), Annelids, and Sipunculoids (P//((800/(I-_\'0I1l(I). The peculiarity of this mode of division is that after the ’r'our-celled stage the blastomeres usually known as the 1nacromeres—give elf ‘quartettes’ of micromeres towards the animal pole, the first quartette being given off’ dexiotropically (except in cases oi.’ reversed cleavage),
 
 
FIG. 13.— Diagram of a ‘ spi1'a‘-Jy ’ segmenting egg in the 16-cell stage. 2 A-2 D macromeres; 2 «:2 d nueromeres of second quartette; ] n 1, 1 n LL Id 1, 1 cl 2 nncronieres of first quartettc.
 
the second laeotropically, and so on in regular alternation, until four quartcttes have been produced. The cells of each quartette divide meanwhile in conformity with the same law of alternation of direction of cleavage. The direction of division is thus always oblique to the egg-axis, and this ()l’)ll(lflll]_Y can be observed in the division of the first two blastomcres, the result of which is that of the two sister cells A and B A is nearer to the animal pole than B, while in the other pair C is nearer that pole than D; A being to the left of B and C to the left of I) (to an observer standing in the axis with his head to the animal pole), the division is laeotropie. The arrangement of cells approaches the tetrahedral, especially when, as occurs very frequently, A and C are united by a cross, or polar, furrow above, B and D by a polar furrow at the vegetative pole, as in JVc=rci.v, Icfinoc/zitou, Limaw, Plano)‘/153, Lejii/Ionolus, Dixcocelis, and others. In Univ, however, it is B and D that are in contact at the animal, A and C at the vegetative, pole. In other cases
 
(I/yauassu, Capi/cl/a, Umbra!/a, Clmyiirlzz/a, Amp/u'6m'te, A/‘euicolu, for instance) the furrows are parallel, the same two blastomercs, B and D, being in contact at both poles. In Troc/ms both the ‘parallel furrows’ and the ‘ crossed furrows’ conditions are found. A similar disposition is to be observed amongst the mieromeres of the first quartette. These mieromeres, also, alternate with the macromeres. Not more than three contact surfaces between blastomeres, therefore, meet in one line.
 
The eggs of certain Lamellibranchs—-Ykrerio, Cg/clas—i11 which the ‘spiral’ arrangement is obscured by the large size of the D maeromere, and possibly the ova of the Rotifera, are to be referred to this type.
 
The tetrahedral arrangement of the first four cells is conspicuous in Asteroids and Ophiuroids, where the planes of division of the first two cells are at right angles to one another. Before the next division, however, the cells shift their positions and come to lie in one plane, in which, however, the sister cells are not adjacent, but opposite, to one another.
 
The eggs of Amp/u'0.mzs sometimes segment spirally (Wilson).
 
After the completion of the spiral period of division, segmentation beeomes radial and then bilateral.
 
3. The third type of cleavage is the bilateral. The first two divisions intersect in the axis; the next. may he equatorial, as in Aseidi-ans. In this case the hilaterality becomes evident in the succeeding phase, in which the divisions in two adjacent cells of the animal hemisphere meet the first furrow, while in the other two they meet the second. The bilaterality is marked in the reverse way in the vegetative half of the egg. The egg is thus divided into what will be anterior and posterior, dorsal and ventral, and right and left halves. In future divisions the bilateral symmetry is retained.
 
The egg of ./Imp/ez'o.avzz.s' may divide in this fashion (Wilson), and this is the normal method, according to Roux, in Ram: esculwz/a.
 
In the Teleostei and some Ganoids (Lepz'do.w‘ws) the bilaterality becomes evident in the third division, which is parallel to the first, the fourth being parallel to the second division. The egg is in fact iso-bilateral.
 
The Ctenophore egg also possesses two planes of symmetry, for the third division is meridional and unequal in such a manner that the next stage~—eig11t cells—is composed of two opposite pairs of small and two opposite pairs of large cells.
 
The mesomeres in the sixteen—celled stage of Ecliinoids are bilaterally arranged.
 
In the Cephalopoda the egg is large-yolked, and segmentation consequently meroblastic. After the firsttwo meridional (livisions
 
V‘ I1.’ I 01' 1’ Flo. 14.—-Tln-ee segmentation stages in the blastoderm of S'I—'pi(I o_)_7i 4-inalis; the segmentation is of the bilateral type. I, left; 2-, right; I— V,
 
first to fifth cleavages. The top sides of the fig111‘es are anterior. (After Vialleton, from Korschelt and Heider.)
 
the bilateral disposition sets in, for the furrows of the third phase are unequally inclined to the first furrow in two halves—— the future anterior and posterior halves—oE the egg (Fig. 14). The egg of A.s'aa7'2's megalocep/m/a also exhibits a bilateral cleavage, but not on the plan just described. The iirst division is equatorial. Then the animal cell divides meridionally, and, as it will prove, transversely, the vegetative cell latitudinally.
 
 
Before the next division the most vegetative cell (P._,) slips round to what will be the posterior side. All four cells are bilaterally arranged about the plane in which they all lie, and this will become the sagittal plane of the embryo. The anterior and posterior ends, and therewith the right and left sides, are likewise now determined. The bilateral symmetry is preserved in future divisions, at least in the vegetative hemisphere ; in the animal part of the egg the blastomeres of the left side become t.ilted forwards, those of the right side backwards (Fig. 155, p. 255).
 
4. In the Triclad Turbellarians, in Trematoda and Cestoda, segmentation is irregular, the blastomeres separate from one another and lie amongst the yolk-cells. The same phenomenon may be witnessed in the Salps, and the separation and subsequent reunion of the blastomeres has also been described in Coelenterates and in Asteroids.
 
Although these types of segmentation are distinct enough from one another, intermediate conditions are readily found. The radial easily passes into the spiral type for example, for in many eggs of the former kind the ‘cross furrows ’ have been observed at either one or both poles, while the animal blastomercs may be rotated slightly on the vegetative, and so lie not over, but in between, them. The radial symmetry again may become bilateral, as when the meridional furrows of the fourth phase, instead of passing through the animal pole, meet the first or second furrow, symmetrically on either side of one of these divisions; this occurs as a variation in I?ana_/'/mew and (normally (Roux)) in Ifamz excz/Zr’):/rt.
 
In ()phiuroids and Asteroids the tetrahedral arrangement is lost, and the egg segments radially. In Amp/riuamr: all three types occur.
 
All three forms may therefore have been derived from one, though what that one was we do not know. In any case, however, one feature is common to them all; in all cases successive divisions are at right angles to one another. This is the law formulated by Sachs long since for the divisions of the cells of plants. It holds good for the segmenting animal ovum, though exceptions may, of course, be found. The alternation of dexiotropic and laeotropic divisions, for instance, in spirally segmenting ova continues for a long period with striking regularity, and it is comparatively rare for a cell to disobey the rule. The rule is, however, no universal law of cell—division. Every embryologist will recollect the continued division of a teloblast in the same direction to form a germ-band, which is such a coilspicuous fact in the development of Molluscs, Annelids, and Arthropods. The four polar nuclei of Insect eggs, lying in one straight line, may also be cited.
 
The direction of division and the size of the blasfomeres are not, however, the only factors which determine the actual pattern of segmentation. The cells can, and do, shift their positions on one another. This is of common occurrence, and a few examples will suflice. The rearrangement of the tetrahedrally disposed cells in Asteroids and Ophiuroids has been ‘noticed already. In many ‘spiral’ ova the micromeres have been observed to rotate on the macromeres, or one quartette to be pushed out of position by the cells of another. In z1.vca;'is the cell P._, slips to one side. 14‘urther, cells change their shape.
 
Two factors are therefore involved in the production of the pattern of cleavage, the direction of division, and the movements of the cells, and these factors in their turn demand explanation. To these must be added the shape, the size, and the rate of division of the cells.
 
The two latter depend very largely upon the amount of yolk present in the egg ; yolk-cells are large, the yolk divides slowly, or not at all. This was expressed long sinee by Balfour in the formula, ‘ The Velocity of segn'1enta,tiQn in ‘ FIG. I5.—' Segnientation Oi‘ the
 
, . Lro0“s one under the influence any Part 01 the ovum 15) roughly of atbeentliijfiigal force (from Korspeaking, proportional to the eon- sc_he1t and Heider, after 0. Hertcentration of the protoplasm there ; ;l(')1(’l'2l'_l111 gzfilcolilmllsitvsijl 3131: and the size of the segments is (yolk-syneytiuin) 2 I.-h,bln.stocoel; inversely proportional to the eon- ”” y°lk'““°1°17 "’ y 011"’ centration of the protoplasm.’ ‘ The rule has been vindicated by (). I-[ertwig experimentally. If the egg of the Frog be centri ‘ ("omp. Emb. i. C. 3. 30 CELL-DIVISION AND GROWTH II. I
 
fugalized with suflicient force the yolk is driven still more towards the vegetative pole, while the protoplasm is accumulated in the animal half of the egg. Such eggs segment meroblastically, a cap of cells or blastoderm being formed lying on the surface of a nucleated but undivided yolk. The yolk-nuclei, moreover, are enlarged, as in megalecithal fish eggs (Fig. 15).
 
The rule is, of course, only applicable to telolecithal eggs, and for many of these it holds good, notably for Vertebrates. In other classes there are, however, exceptions, which are best known in those \vhose segmentation has been most carefully studied, the ‘ spiral’ eggs of Turbellarians, Annelids, and Molluscs. Large cells, in these ova, often divide more quickly than small ones; the second quartette of micromeres, for instance, is formed before the first quartette divides in 07-epitlula, Uuio, Limaw, Troc/ms, Aplg/sia r/epilans, Discocelis, and the cells of the third quartette before the first products of division have had time to divide again in L[maa', Umbrella, and A/2/ysia limacina. 411 is often formed before the corresponding cells in the other quadrants (in Ifizio, for example), but in Crepirlula this is in accordance with the rule, since 4 a, 4 /2, and 40 contain more yolk than 4« cl. In Arem'coZa, though the yolk is uniformly distributed, the cells are still unequal. Other exceptions are to be found in the continued unequal division of teloblasts, in the formation of the micromeres in Echinoids, and in the unequal division of the blastomeres in the third and fourth phases in Ctenophors. According to Ziegler the formation of the micromeres in Ctenophors cannot be due to the presence of yolk, since they are still formed when part of the vegetative hemisphere is removed, as Drieseh and Morgan have also found.
 
Ziegler indeed puts forward another hypothesis to account for unequal division; he supposes that the centrosomes are heterodynamic. So far there appears to be little evidence in support of this view. It is quite true that in many cases of unequal division the asters—not the centrosomes——-vary in size with the size of the cells. This occurs, for instance, in the division of the first micromeres and of the first somatoblast in Nerei.9, in the formation of the first and second quartettes, and in the division of the first somatoblast in Uuio, in the division of the cell CD in /laplauc/ma, and in the division of the pole cells of Annelids (Wilson and Vejdovsky). It is doubtful, however, whether it is not the inequality in the cells that is responsible for the inequality of the asters, there being more room in a large cell for the outgrowth of the astral rays. At any rate, ‘there are many cases of unequal clcavage——in polar body formation~——where the asters are of the same size. Until evidence is brought forward of a difference in the size of the centrosomes the hypothesis is no more than a conjecture.
 
Before quitting this subject we should refer to a rule which Zur Strassen has found to hold good for the rate of segmentation of Ascaris megalocep/zala. The cells do not all divide at the same rate, but in certain groups of cells division is found to occur simultaneously. These cells are related, descended from some one cell, and the more nearly related the cells are, the more nearly together do they divide. Coincidence in time of division depends therefore on the degree of cell-relationship.
 
The direction of division of the cell depends upon that of the nucleus, since, speaking generally, the division occurs in the equatorial plane of the spindle, or, in other words, the plane of division is at right angles to the direction of elongation of the spindle or separation of the ccntrosomes. The latter again depends on the relation between the nuclear spindle and centresomes on the one hand, and on the other the cytoplasm and its contents, more particularly the yolk. The relation between the (resting) nucleus and the cytoplasm has been expressed by O. Hertwig in the following empirical rule: ‘The nucleus always seeks to place itself in the centre of its sphere of activity.’ The sphere of its activity being not the inert yolk but the cytoplasm, we find, in accordance with this rule, that the nucleus places itself in the centre of the egg where the yolk is uniformly distributed (isoleeithal), nearer the animal pole but still in the axis where the yolk is on one side (telolecithal). Examples of the former condition are to be found in Eehinoids (the fertilization nucleus is nearly, but not quite, central) and large-yolked Arthropod ova, of the latter in the eggs of Vertebrates, Molluscs, and many others. The nucleus, however, may wander from this position, as occurs, for instance, in the egg of Eehinoids after the expulsion of the polar bodies and before fertilization. Apart from such exceptions, due very likely to some temporary alteration in the relations of yolk and cytoplasm, the rule is a reliable one.
 
The relation between the dividing nucleus, the spindle and centrosomes and the cytoplasm has been stated by O. Hertwig in his second empirical rule ‘that the two poles of the division figure come to lie in the direction of the greatest protoplasmic mass ', by Pfliiger in the l'o1-mula, ‘the dividing nucleus elongatcs in the direction of least resistance.’
 
The objection that has been urged to this latter expression, that in a fluid the pressure is equal in all directions, may be set aside. For though the cytoplasm is fluid it is an extremely viscid fluid, and the presence of the suspended yolk granules
 
 
 
FIG. 16. — Diagram oi the segmentation of the Frog's egg (after 0. llertwig, from Korschclt and Heider). A, first (meridional); B, third (latitudinal) phase of segmentation; p, superficial pigment of animal hemisphere; pr, protoplasm; y, yolk; sp, spindle. must certainly ofl'er a greater resistance than the fluid itself, and greater in proportion to their number and size. Pfliigcr’s formula, therefore, if not merely a truism, resolves itself into a restatement of Hertwig-’s rule. This rule certainly holds good for a large number of cases, for it explains, for instance, the two first meridional divisions of all spherical telolecithal and radially segmenting eggs, the third, latitudinal (in smallyolked eggs 1), and possibly also subsequent meridional and latitudinal divisions (Fig. 16). It will not, however, in the present state of our knowledge, explain the obliquity of the spindles to the egg-axis in spirally dividing ova, nor cases of bilateral division ;
 
1 ln Sycon the third is meridional, the fourth latitudinal. II. I CELL-DIVISION 33
 
here, it is evident, other factors must come into play, in the second case probably a bilateral symmetry in the constitution of the cytoplasm. These exceptions may, however, ultimately prove to be special cases of Hertwig’s rule.
 
A very striking confirmation of the rule is to be found in the division of the egg of Arcane m7_//roreuoxa (Figs. 17, 18). The egg of this worm is ellipsoid. At one end (that turned towards the upper end of the ovary) the polar bodies are extruded, and here the female pronucleus is placed. The spcrmatozoon enters at the opposite end. The line of union of the two pronuclei therefore lies in the long axis of the egg. Nevertheless the fertilization spindle is not formed in the minor axis of the ellipsoid as one might expect. The two pronuclei rotate together through 90°, the spindle is developed, as usual, in a direction at right angles to their line of union, that is to say the axis of the spindle lies in the major axis of the egg, and the rule is confirmed. There is a similar rotation of the fertilization spindle in the egg of another Nematode, Diployaster (Ziegler), and in the Rotifer Airplane/ma the spindle, at first oblique, becomes later coincident with the long axis of the ovum (Jennings).
 
 
 
 
FIG. 17.- Four stages in the fertilization of the egg of A.»-z-(n-is m'_qroveuosa. (After Auerbach, from Korschelt and Heider.)
 
 
 
 
Fig. 18.- Tl1ree diagrams of the rotation of the fertilization spindle in the egg of Ascaris nigrovenosa. e, s, the directions in which the female and male pronuclei approached one another in A; 1, 2, 3, successive positions of the spindle. (From Korschelt and Heider, after 0. Hertwig.)
 
 
 
Curiously enough, this rotation of the pronuclei does not occur in another ellipsoid egg, that of the Rotifer Callidiua. According to Zelinka, the polar body is formed at one end of the long axis, but the fertilization spindle lies in the minor axis, the first division includes the major axis, and the law is disobeyed. After the division, however, the cells rotate, and the plane of contact is then, as in Ascaris m'groveuo.m, transverse.
 
Again, all polar divisions violate the rule, as also does the first division of the fertilized egg of Away-is megalocep/zala, and the division of the cells of the germ-bands of Crustacca parallel to their length (Bergh).
 
On the other hand, Ilertwig has brought forward experimental evidence in support of his generalization. In the eggs of the Frog the directions of some of the divisions were altered by compression between glass plates. The eggs were just allowed to assume their normal position with the axis vertical. They were then placed between glass plates and compressed.
 
In the first series of experiments the plates were horizontal. In such eggs the first furrow was meridional and vertical, the second meridional and vertical and at right angles to the first. So far, therefore, division was as in the normal egg. In the third division the furrows were, however, not latitudinal and horizontal, but nearly vertical, being parallel to the first furrow above, to the second furrow below. The surface of contact, therefore, formed by the furrows of this phase must pass through a meridional position in the interior of the egg. The fourth furrows are latitudinal. Born has repeated the experiment and confirmed this result (Fig. 19). He adds, however, that the furrows of the third division pass II. I CELL-DIVISION 35
 
towards the vegetative pole below, or may even remain parallel to the first furrow throughout. The fourth furrows, Born says, are parallel to the second. I have myself observed that this division may be either parallel to the second, or latitudinal, even in different quadrants of the same egg (Fig. 20). It will be observed that the quadrant in which the third furrow is latitudinal is smaller than the others. It is of great interest to observe the striking similarity between the direction of the third and fourth furrows in these eggs and the corresponding divisions in the Teleostean egg where the blastodisc is compressed by the chorion.
 
 
FIG. 19.—- Segmentation of the Frog's egg under pressure. The compression is in the direction of tlie axis. A. v1ew of the egg between horizontal plates; the animal part IS
 
islraxled. II, (I, I), first (1), second (2), third (3), and fourth (4) divisions as seen from the animal pole. (Alter Born, from Korsehelt and lleider )
 
 
 
FIG. 20. — The first four divisions (I, II, III, IV) in a F10g’s egg compressed between horizontal plates in the direction of the axis. The third furrow is more or less meridional and vertical in three quadrants, horizontal in the fourth, and this a smaller quadrant. 'l‘he fourth furrow is meridional in this quadrant, horizontal in the remaining three.
 
 
 
In the second series of experiments made by Hertwig the glass plates were vertical, the eggs, therefore, compressed not, as before, in, but at right angles to, the axis.
 
The first furrow was meridional, and therefore vertical, and at right angles to the plates. The second was latitudinal and horizontal, and also at right angles to the plates. The furrows of the third phase were parallel to the first, those of the fourth, in the four upper animal cells, parallel to the plates. Born again
 
 
 
F1G.2l. — Segmentation of the Frog's egg under p1'ess1u'e. The pressure is at right angles to the axis.
 
A, view of the colnpressed egg. The piglnented animal portion is shaded.
 
If, C, 1), views of the egg from the animal pole after the first (1), the second (2), and the third (3) divisions.
 
E, 11‘, 0, views of the egg from the compressed side after the first (1), the second (2), the third (3), and the fourth (4) divisions. The first furrow may pass as (1') in E.
 
(From Korschelt and Heider, after Born.)
 
confirms this account (Fig. 21). The direction of the furrows of the third phase is, however, variable ; it may be not parallel to the first, but perpendicular to it. In this case it may be parallel to the second, or so oblique to it as to become nearly parallel to the glass plates. The direction of the fourth division depends on that of the third, to which it is at right angles. It may, therefore, be either oblique and nearly parallel to the plates, as described by Hertwig, or parallel to the second furrow and perpendicular to the plates. In a third series of experiments I-Iertwig placed the plates obliquely, at 45°. In these eggs the yolk sinks slightly from the upper to the lower side, while the cytoplasm rises in the opposite direction; in other words, a bilateral symmetry is conferred upon the egg by the combined action of pressure and gravity. The plane of this symmetry is midway between and parallel to the plates. The first furrow is at right angles to the plates and to the plane of symmetry.
 
VVe are indebted to Drieseh for a similar series of experiments on Echinoderm eggs. Drieseh compressed the eggs of 15?:/ri/u/.s~ under a cover-glass supported by a bristle. The direction of the egg—axis with regard to the pressure was not known, but the
 
 
 
FIG. 22.——I9'rIu')ms: segmentation under pressure.
 
(1, preparation for third division (radial); b, preparation for fourth
 
division (tangential); 11', after fourth division; 0, another form of the 8-cell stage (third division pa1'allcl to first); «I, the same after removal of the pressure. (After Driesch, 1893.) Echinoid egg is nearly isolecithal. VVhen the egg membrane remained intact the first two furrows were vertical, that is, in the direction of the pressure, since the slide and cover-glass were horizontal, and at right angles to one another.
 
The spindles for the next division are again horizontal, and usually tangential, sometimes, however, radial. The eight-celled stage consists, therel'ore,o[’ a flat plate of cells. At the next division the formation of mieromeres —which would ordinarily occur at this moment——is suppressed; the spindles are horizontal and radial, the furrows, therefore, vertical and tangential (Fig. 22 a, /1, /2’).
 
In certain cases cell-formation is wholly or partially suppressed. When the pressure is less (in those eggs which lie nearer the bristle) the micromeres may be, but generally are not, formed. The spindles are no longer horizontal. Similar results are obtained when the eggs are released from strong compression.
 
In another experiment the eggs were first deprived of their membranes. The first and second furrows are vertical and generally at right angles to one another. Sometimes,-however, 38 CELL-DIVISION AND GROWTII II. I
 
the second is parallel to the first, or one blastomere may lie apart from the other three. Should the eggs be now released from the pressure, each blastomere becomes rounded off, and——after two more cleavages—-the sixteen-celled stage consists of two plates of eight eells lying over one another. But if the pressure is maintained, the spindles are horizontal and the blastomeres lie all, or nearly all, in one plane (Fig. 22 c, (7).
 
 
 
Fig. 23.--Segnieiitatioii of the egg of E(‘7li1lII8 micI'oh¢bcrcuIulus under pressure. (After Ziegler, 1894.)
 
(V, 8 cells in one plane; 1;, 16 cells, the last division having been tangential ; c, (I, 16-32 cells: the direction of the spindles in c is shown by the line: it is in the greatest length of each cell; c, 64 cells: a cross signifies a vertical or oblique division, a line a horizontal division.
 
Ziegler has followed the segmentation of the compressed eggs a step further (Fig. 23). As the figures show, the first two divisions are at right angles to one another, while the furrows of the next two phases are, roughly, parallel to the first and second. In the next division-—sixteen to thirty-two cells—the outer cells divide radially, the inner more or less tangentially, these divisions being, like the previous ones, at right angles to the compressing plates. In the following phase, some cells (those marked with a line) still divide in the same direction as before; but in others (distinguished by a cross) the spindle is perpendicular to the II.
 
plates and the division horizontal. Ziegler points out that, in the former cases, the cells have greater dimensions in the horizontal plane than in the la.tter. This, however, may be the efieet, not the cause, of the direction of the spindle-axis.
 
Two other pressure experiments maybe mentioned here. In Nereis Wilson produced a flat plate of eight equal cells by applying pressure in the direction of the axis. The formation of the first quartette of micromeres was thus suppressed. On relieving the pressure eight micromcres were formed. For the Ctenophora (]ierb'z') Ziegler has shown that the normal inequality of the third and fourth divisions is not altered by pressure.‘
 
The foregoing experiments all agree in demonstrating the perfectly definite eft'cct produced by pressure upon the segmenting cgg. The nuclear spindles place themselves at right angles to the direction of pressure, the divisions fall at right angles to the compressing plates. This holds good for the first three or four divisions, at least, and sometimes for later phases still. In all these cases, therefore, the nuclear spindle elongates in a direction of least resistance, and, in the normal uncompressed egg, we may argue, with Ilertwig, the least resistance is offered by the greatest protoplasmic mass.
 
Even in the compressed eggs, however, the greatest extension of the protoplasm, or the least extent of the yolk, is a factor which must in some cases come into play. When the egg of the Frog is compressed between vertical plates, the nuclear spindle does not elongate in any direction at right angles to the pressure, but in one only, a horizontal ; and this is the direction of the greatest protoplasmic mass, since the egg-axis is vertical.
 
Speaking generally, therefore, experiment has upheld Hertwig’s contention that the direction of nuclear division, and therefore of cell-division, is determined by the relation between the nucleus with its centrosomes and the cytoplasm with its yolk.
 
There are one or two experiments which do not support Hertwig’s view. Boveri stretched the eggs of the sea.-urchin St7'o2z_9;yloceul/'0!/1.9 in the direction of the axis. The fertilization spindle lay in the usual equatorial position, occupied, that is, the minor axis of the ellipsoid.
 
‘ I have recently had occasion to notice that when the egg of Anfedm is compressed in the direction of the axis the third division is meridional instead of latitudinal.
 
Again, Roux observed that Frogs’ eggs sucked up into a tube with a narrow bore became elongated either parallel or transverse to the length of the tube, the axis of the egg lying in each case lengthways. In the first case the division was at right angles to, in the second usually parallel to, the tube in accordance with the rule; but exceptionally, in the transversely stretched eggs, the division was not perpendicular to, but coincided with, the extent of the greatest protoplasmic mass.
 
However important a factor the disposition of the yolk may thus be in deciding the direction of cell-division, it is certainly not the only factor. In the eggs pressed between horizontal plates there are many—an infinite number—of' directions of least resistance. In one of these the segmentation spindle elongates, and at right angles to this the first furrow falls. This is probably determined-——-as it is determined in the normal Frog and Sea-urchin egg-——by the point of entrance of the spermatozoon, or at least by the direction of the sperm-path in the egg. The second division is at right angles to the first, and here the direction may very possibly be decided on lIertwig’s principle. But why, in the next phase, should the furrows be at right angles to the second rather than to the first, for the extent of the protoplasmic mass is as great in each of the four cells, in a direction parallel to the first as to the second furrow? Here, it is clear, some other reason must be found for this succession of divisions at right angles to one another. The cause is probably to be sought for in the direction of division of the centrosomes ; for these divide——frequently soon after the telophase—at right angles to the axis of the previous figure. VVe thus gain a new expression for Sachs’ Law.
 
The original direction of divergence of the centrosomes is, however, by no means always the ultimate one, for the growing spindle may be twisted out of its original position. Conklin has made a careful study of this phenomenon in C’)'¢j[Ii(]l(ltl, in which egg he finds that vortical movements are set up in the cytoplasm by the escape of nuclear sap at the beginning of mitosis. The movements are in opposite directions in sister cells, centre in the spindle poles, and often carry both nucleus and spindle into a fresh place. These currents, which had been noticed previously by other observers (by Mark in Limam and by Iijima in Ale]//zelis), may thus play an important part in the production of the cell pattern. We shall see elsewhere that they, and other protoplasmic movements, are also of the very greatest significance in difierentiation.
 
There remains now to be noticed another principle, which is especially applicable to plant-cells with fixed walls, though it may possibly be used for the phenomena of animal segmentation as well. Berthold has pointed out that when a newly formed cell-wall places itself perpendicular to the previously existing walls it is——at least in a good many instances—simply obeying tlie laws of capillarity, it merely conforms to the principle of least surfaces formulated by Plateau. This principle is as follows: ‘ Homogeneous systems of fluid lamellae so arrange themselves, the individual lamellac adopt a curvature such that the sum of the (external) surfaces of all is under the given conditions a minimum.’
 
A fluid lamella, of soap solution, for example, placed across the interior of a hollow, rigid cylinder, or parallelepiped, or cube, is, with the film coating the internal surface of the vessel -in which it lies, a special case of such a system of lamellae, and, in obedience to the principle, the lamella places itself at right angles to the walls of the cavity and transverse to the long axis.
 
In the ease of the plant-cell, the cell-plate, formed by solidification of the spindle fibres in the equator of the mitotic figiii'e, represents the soap-lamella, and like the latter in its parallelepiped, the cell-platc, or new cell—wall, places itself perpendicular to the old one, and transverse to its length.
 
There are very numerous cases in which the law is obeyed, but it is not so in all. Under certain conditions the. lamella should be not at right angles, but oblique to the wall of the chamber across which it is stretched. If, to take a concrete case, the lamella be made to move (by abstracting air) towards one end of its receptacle (a cube or parallelepiped), it will reach a critical position in which the principle of least surface can only be satisfied by its occupying an oblique position. The
 
point at which this occurs is when the lamella is distant 3 from 7r
 
the end, where a is the length of the side of the cube (short side of the parallelepiped). The lamella new forms the fifth side
 
to a wedge-sliaped space (quadrant of a cylinder, whose radius
 
. 4 . . . . is 1- = 9-a), but as more air is abstracted, and it moves still further toward the end, it comes to another critical position when it must lie across one corner, forming so the base of a pyramid, or octant of a sphere. This position is defined by the equation 2-, =11, where 1', is the radius of this sphere. It is impossible, therefore, for a very fiat cell, or short cylinder, to be divided in conformity with the principle parallel to its longest side, and yet this occurs, as, for instance, in cambium cells.
 
It will also be noticed that this principle does not explain why one particular direction is selected when many are apparently equally possible.
 
We turn now to a consideration of the remaining factor which assists in determining the shape of the cells and so the geometrical pattern of segmentation ; this is the movement of the cells upon one another.
 
That such movement does occur we have already seen; the question which immediately suggests itself is whether in taking
 
up their new positions the cells obey the laws of capillarity as enunciated for systems of fluid lamellac such as soap-bubbles by Plateau in his principle of least surfaces.
 
This principle, as we have seen, demands that the sum of the external surfaces should be, under the conditions, a minimum, or, expressed in physical rather than in geometrical language, that the total surface energy should be minimal. In accordance with this doctrine of minimal surface energy a drop of fluid floating in a fluid medium assumes, as need hardly be said, the form of a sphere. In a system of drops contact surfaces will be formed between the drops, provided that each possesses a coating film which has a positive energy with the media it separates; a film, that is, of such a nature that the total surface energy would be diminished by apposition, without, however, involving the disappearance of the separating film and fusion of the drops. In other words, the film must be insoluble in both the external and the internal media. A simple example of this is afforded by the behaviour of the spheres of jelly covering the eggs of the Frog, when taken from water and floated between chloroform and benzole. Two or more such drops of jelly cohere by their coating films, and form systems of lamellae —the films, that is, at the external surfaces and between the opposed surfaces of the drops———in which the principle of least surfaces is obeyed. Soapbubbles form similar systems. But where this condition is not fulfilled, as in oil-drops floated, for instance, between alcohol and water, the drops either unite or separate, each retaining its spherical form.
 
The geometrical analysis of such systems given by Plateau is as follows. In a system of two bubbles the curvature of the
 
surface of contact is given by the equation r = £7, where 7' is the radius of that surface, p, p’ the radii of the larger and smaller bubbles. Since the pressure varies inversely with the radius, the surface of contact is convex towards the larger bubble. When p = p’, 7- = a, and this surface is plane. Since there is equilibrium the external surfaces of the bubbles and their common surface meet at angles of 120°.
 
In a system of three bubbles there are three contact surfaces; these meet in one line and make angles of 120° with one another. When there are four bubbles, however, the four con tact surfaces cannot meet in one line except for an inappreciable instant; they immediately shift their positions in such a way that two opposite bubbles meet and separate the other two from one another. There are thus five surfaces of contact, and these make angles of 120° with one another as before. This is the arrangement when four bubbles—whether equal or unequal is no matter—are placed side by side in the same plane. When, however, one bubble is placed in a different plane to the remaining three, four surfaces are formed and disposed in such a manner that the four lines, each formed by the intersection of three of these surfaces, meet in one point, making with one another angles of 109° 28’ 16", the angles at the centre of a tetrahedron. In short, the four are now tetrahedral] y arranged. The systems of drops of jelly alluded to above arrange themselves as do soap-bubbles under similar circumstances. What holds good of four holds good of an assemblage of any number of bubbles. The size of the bubbles is a matter of indifference, except to the curvature of the surfaces of contact, and, to a certain extent, to the arrangement. Thus, if four equal bubbles be placed in a plane, they will form together five surfaces of contact, one of which will be between two opposite bubbles. If these two be now diminished, or the opposite two enlarged, the surface of contact will be between the opposite pair of larger bubbles. On the other hand, it is possible to bring smaller opposite bubbles into contact, while the larger ones remain apart. Again, on four bubbles lying in one plane, four small ones may be superimposed in such a fashion that while two lie at either end of the surface of contact, the other two lie over between the two opposite large bubbles below. If now the two latter small bubbles be enlarged, they will displace the other two until all four come to lie not over but between the
 
 
 
FIG. 24.—Diugra1ns of systems of soap-bubbles.
 
A-0, four small bubbles superimposed on four large ones. In A and B the bubbles are not compressed ; in C the lower bubbles have been circumscribed by a. cylindrical vessel. In B the upper bubbles are small enough to show the surfaces of contact between each and the two adjacent large bubbles below. These surfaces are invisible in A and C.
 
D is a system of eight bubbles in one plane, four forming a cross in the centre.
 
In all figures notice the fifth contact surface or ‘polar furrow ’.
 
bubbles below, the usual arrangement when four are superimposed on four (Fig. 24 A—C).
 
The final disposition must depend, therefore, not merely on the principles of least surfaces, but also, provided that the conditions of that principle are fulfilled, on the sizes and initial arrangement of the bubbles.
 
It will hardly need pointing out that very many ova adopt the form which presents the least external surface, that of a sphere, when placed in a fluid medium, and it is also a familiar fact that after the first (and subsequent) divisions the blastemeres are flattened against one another (Cytarme, to use Roux’s term), and that whether they are compressed by an egg membrane or not (examples of the second alternative are to be found in Unio, .D1'eis3eu8z'a, Umbrella, C'7'q2i(I'/(la, /lp/yxia limecimz, /late;-iae), though the surface of contact is not always curved when the cells are unequal. The two cells, however, often become rounded of and partially separated from one another prior to the next division. Such a separation (Cytochorismus) has also been observed by Roux in the ease of cells of the Frog’s egg, which, having been isolated in albumen or salt solution, have subsequently reunited.
 
That the cells flatten against instead of repelling one another, as free oil-drops would do, suggests that they, like soap-bubbles, are provided with an insoluble coating-film, while their subsequent separation may be provisionally explained by supposing that this coating-film becomes temporarily dissolved under the action of some substance formed in the cell. This idea is borne out by a striking experiment of Herbst’s, who found that in sea-water deprived of its calcium the blastomeres of the seaurchin egg came apart and resumed their spherical shape. At the same time the surface membrane underwent a visible alteration, becoming radially striated. It seems reasonable to conclude that there is a membrane by which contact is normally effected, and that this is soluble in sea-water devoid of calcium. On the addition of calcium the cells eohere again.
 
It may be mentioned that when systems of drops of jelly, floating in a medium of oil and united by their coating-films of water, are removed to alcohol, in which both oil and water are soluble, the films disappear and the drops separate.
 
In the next stage (four cells) the type of segmentation in which the laws of capillarity are most strictly obeyed is obviously that which we have distinguished above as the spiral or tetrahedral type, and Robert has been able to show that successful imitations of the four-, eight-, twelve-, and sixteen-celled stages of the egg of 2’7'oc/we may be made with soap-bubbles.
 
Four equal bubbles were placed in a porcelain cup, which held them together in the same way that the actual cells are held together by the vitelline membrane. Five surfaces of contact were formed, that between two opposite bubbles representing the cross furrow or polar furrow in the egg. In the fl’/'oc/ms egg, however, the polar furrows need not be parallel at the animal and vegetative pole; they may be at right angles to one another, and this tetrahedral arrangement of crossed polar furrows may be imitated by lifting up one of the bubbles and bringing it into contact with its opposite, one pair of bubbles being new in contact below, the other pair above. This arrangement is, however, unstable whilc the four bubbles remain in one plane, the two bubbles soon coming into contact both above and below. When the bubbles are not confined within a cup the instability of the ‘ crossed-furrow ’ condition is extreme.
 
By reducing the volume of the bubbles that are in contact the other two may be brought together; as the polar furrow changes positions there is at least a temporary condition when they are crossed.
 
As we have already pointed out, both conditions—the ‘ parallel furrows’ and the ‘ crossed furrows ’—-are met with in the eggs at the four-celled stage of Molluscs, Annelids, and marine Turbellarians. Whether both opposite pairs or only one opposite pair of blastomeres are in contact does not, however, appear to depend upon whether the vitelline membrane is close to and compresses the egg or not. In most cases of crossed furrows the membrane fits, it is true, quite closely (Nereis, Io/moo/titou, Porlar/cc, Lcpidouotus, Jjiscocelis, P/(yea, and possibly Li)/may and Planorbis, if there is in these two, as in P/13/ea, a very fine membrane between the albumen and the ovum); so also, speaking generally, where the furrows are parallel the membrane is absent (Umbrella, ./ljzlysia, Dreisseueia, Crepirlu/a), but in Am];/aitrile and C/gmzenella it is lightly applied to the egg.
 
It is remarkable that when the furrows are crossed, it is the A and C cells which meet at the animal pole, the B and D cells at the vegetative (except only in U/tio), and this must depend on other properties of the cells than their surface tensions. But it may be very plausibly suggested that the explanation of the fact that it is the cells B and D which meet to make the ‘parallel’ furrows is to be looked for simply in the large size of D.
 
Robert has indeed shown that by simply altering the sizes of
the bubbles the conditions observed in the four-celled stage of other types—Nereis, A/-euicola, Uuio, zlplysia, I)isc-ocelz'.s°-may be faithfully copied.
 
It only remains to be added that the contact surfaces of the cells, like those of the bubbles, make angles of 120° with one another.
 
Robert has also imitated the eig-ht—eelled stage (the four micromeres alternating with the four maeromercs), the stage of twelve cells (division of the micromeres), and that of sixteen cells (second quartettc formed). The bubbles of the second quartette may be made to slide in between the maeromeres and so rotate the whole first quartette, as happens in the egg. The division of the micromeres in the egg results in the arrangement of four cells crosswise in the centre, four others occupying the spaces between the arms of the cross. The bubbles behave in the same manner.
 
In the eigl1t—celled stage the micromeres alternate with the macrorneres. In the case of the bubbles this is not necessarily so; the two sets of bubbles may be superposed if the ‘polar furrow’ in one tier is at right angles to that in the other, or if, as pointed out above, the upper bubbles are small. Otherwise superposition is a very unstable condition.
 
It would appear then that many of the patterns exhibited by eggs with a spiral cleavage are explicable by reference to the laws of surface tension. The principle of least surfaces may be extended to other cases. The first four blastomeres of Ophiuroids and Asteroids form a perfect tetrahedron, though this arrangement is subsequently discarded for one which could not be imitated with soap-bubbles (we may notice in passing that in the first case the egg is tightly invested by its membrane, in the second it is perfectly free). In zlscarzlv megalocep/aala the four cells come to lie, as do four bubbles, in one plane, and polar furrows have been seen in many eggs which belong to another type of segmentation (in Coelenterates (llyrlractiuia), Sponges (Spongillu), Crustacea (Brauc/z2'pu.v, Luci/‘er, 0rc/Iestia), Vertebrates (Petra/1z_yzo:2, Rana), Ascidians, and Am/2/u'oama).
 
The principle of least surfaces——not more than three surfaces meeting in a line, not more than four lines meeting in a point— is, however, not of itself suflicient to explain the whole of the phenomena even in this most favourable tetrahedral type;
other factors must intervene, just as other factors intervene in a mass of soap-bubbles—their size and initial arrangement~— in the determination of the actual pattern. These other factors are the direction of cell-, that is of nuclear, division, and the magnitude of the cells; and these, as we have seen, in turn depend upon the relation between the nucleus and the cytoplasm with its included yolk. Thus it is the direction of the spindles which determines whether the mieromeres of the first quartette shall be given ofi laeotropieally or dexiotropically ; the direction of division, oblique to the egg-axis, again determines that the mieromeres shall alternate with the macromeres and not be superimposed upon them ,- the size of the cells and the direction
 
 
 
FIG. 25.-— Mitotic division with elongation of the cell-body in a protozoon,Acanthor_1/stis aculeuta. (After Schaudinn, from Korsehelt and Heider.)
 
of division may determine the position of the polar furrow, while the rate of division will also not be without effect, since the whole arrangement at any stage depends in part on the disposition at the stage before.
 
There is one other point that is worthy of notice. The mitotic spindle possesses considerable rigidity, and is able as it elongates to materially alter the shape of the cell. This may be seen in many cases in Annelid, Mollusean and other eggs—the division of the first mieromeres in Nereis is an instance—and in the Protozoa (Fig. 25). Another interesting case is the Rotifer Asplcmc/ma, where, preparatory to the fourth division, the shortest axis of the cells——in which the spindles are placed-—becomes by the elongation of the spindles the longest. This alteration of shape is itself an important factor in deciding the positions to be taken up by the daughter cells. II. I CELL-DIVIS ION 49
 
In the other types——radial and bilateral——thc principle of least surfaces is obviously disobeyed, for here four or more surfaces meet in one line and at angles other than 120°.
 
Roux (1897) has, however, shown that if a certain condition be imposed on the system of lamellac, figures may be produced which very closely resemble the patterns presented by radially and bilaterally segmenting ova. This indispensable condition is that the system shall be surrounded by a rigid boundary, as the eggs themselves are by a membrane. Roux’s system was made by dividing into two, four, and eight a drop of paraflin oil suspended in a closely fitting cylindrical vessel between alcohol and water. To this medium was added calcium acetate to prevent the drops reuniting. The drop was divided with a glass rod.
 
 
Fm. 2(5.~ltoux's oil-drops. A and B, the drop divided equally; U and D, unequally. Each of the two equal drops divided equally in E, unequally in 1". (From Korschelt and Heider, after Roux.)
 
When the two drops formed by the first division were equal the surface of contact was flat, when unequal convex towards the larger one, in accordance with the rule (Fig. 26 A—l)).
 
When the second was also equal, four drops were formed with four surfaces of contact meeting in one line, or enclosing between them a small ‘segmentation’ cavity. If the division of the two equal drops was unequal, and the smaller cells adjacent, they pushed into the larger ones; the result, in fact, was the same as would have been produced by an equal following on an unequal division, the four surfaces meeting in one line as before (Fig. 26 E, F). The appearance presented is like a side view of a radially
 
 
 
Fm. 27.—-Arrangement of four oil-drops produced by unequal division of two equal drops, the small and large drops alternating. The first division is shown by I: the second (II) may pass as in a or in b, but the result is always as in c, the two large drops meeting in a polar furrow and excluding the small drops from the centre; the system is symmetrical (iso~bilateral) about the dotted lines in c. (After Roux, from Korschelt and Heider.)
 
segmenting egg after the third division. When, however, the smaller drops were 11ot adjacent, but opposite, five surfaces of con ” I 3 /. tact were formed,
 
a polar furrow appearing between
 
A A the two larger and
 
I L 1 joining the centres \ /7 W of mass of the two smaller drops,
 
I I whether these are unequal or not.
 
C’ . . The direction 1n
 
fi which the division
of the drops is per” V 3’ formed isirrelevant; the final result is 4% always the same.
 
/ Should two adjacent
 
_lfIG. 2S.-— A_and B are diag1':1111s of an oil—drop drops be equal, the divided into four and eight to explain Roux’s 1 f _ - - 1 notation. C is a figure of the oil-drop divided into P0 M m row 15 Sh]
 
eight equal parts. (From Korsclielt and Heider.) formed by the union of those two which have together the larger mass (Fig. 27).
 
 
The length of the polar furrow varies directly with the size of the drops which unite to form it; its direction makes an angle with the plane separating the first two, which varies
 
 
 
FIG. 29. ~ Arrangenicnts of six oil-drops. In all cases A = B = a = b. In A, rt’ = a", b’ = b". In B, a’ > a", b’ > b”. In C, a’ < a”, b’ < b”. I, first furrow; II, second furrow. (From Korsclielt and Heidcr, after Roux.)
 
 
 
FIG. 30. - Various arrangements of eight oil-clrops, all bilaterally syiiiinetrical about the first furrow (1). In all cases the first division has been equal. In A and B the second division (II) has also been equal, but in C 0, b are smaller than A, B. In A, a”, la”, A”, B" < rs’, b’, A’, B’. In B and C, a”, b" < a’, b’, but A", B” = A’, B’; hence a”, I)" < A", B". (From Korschelt and lleider, after Roux.)
 
 
 
_Ei_G. 31. —~ Arrangement of six (A) and eight (13) oil-drops, after iineqnal division of four equal drops (A = B = a = 1;), the smu.llcr and linger drops regularly alternating. (From Korschelt and Hcider, after Roux.)
 
 
 
inversely with its length, so that when all the drops are equal the cross furrow lies in the same plane with the first division, and so disappears.
 
By another division it is possible to make a ring of eight drops whose surfaces of contact all meet in one line, or in a ‘segmentation’ cavity (Fig. 28). To realize this condition, however, it is necessary that the division should be equal, and its direction accurately radial. If unequal, the larger drop invariably passes towards or wholly into the inside. If oblique or tangential the inner drop passes into the segmentation cavity (Fig. 32).
 
 
 
FIG. 3'Z.——'l‘hree stages in the passage of a large drop (a") into the centre of the system. The fiist stage extremely unstable. (From Korschelt and Heider, after Roux.)
 
Unequal division of all four equa.l drops produces very interesting patterns, some of which recall the appearance of bilaterally segmenting ova, when the divisions are corresponding-ly unequal on each side of the first or second division (Figs. 29, 30), while others resemble certain phases of ‘ spiral’ division when small and large cells regularly alternate (Fig. 3]). It is a rule for the smaller of the two drops to go to the periphery, while the larger assumes an oblong or wedge shape, passing towards the centre if it does not slip entirely inside. The latter occurs with clean oil, when the large drop is flanked by small ones on both sides.
 
It is also possible to divide four equal drops horizontally into two tiers. The upper drops, however-—-unless absolutely undistnrbed~quiekly come to alternate with the lower.
 
In these systems of drops the final arrangement is due to, first, the principle of least surfaces; secondly, the circumscribing boundary; thirdly, the size of the drops; and fourthly, in some cases, the direction in which they are divided.
 
It only remains for us to consider, with Roux, to what extent the cells of a radially segmenting egg, such as that of Rana fusca, are governed by the same influences as determine the pattern of the drops.
 
The resemblances, it will be conceded, are often very close. There are also important differences. The polar furrow, which is often present in the Frog’s egg, is not necessarily between the cells with the greatest mass. Again if, in the four-celled stage, with no polar furrow, one of the cells be diminished by puncture, a polar furrow does not always appear, as it would with oil-drops, nor, if it does, is it always formed by the union of the larger cells. 01', if when a polar furrow is present between the larger cells, one of these is diminished by puncture until it, together with its opposite, is less than the other two, the polar furrow nevertheless retains its position.
 
In the sixteen-celled stage the animal cells together form a ring of eight around the axis. The cells are not necessarily equal, and a small cell may be compressed by, instead of compressing, adjacent large ones, while they, not it, move away to the periphery.
 
Other differences are that large cells bulge into small, that cells are elongated tangentially instead of radially, that there are amoeboid processes at the inner ends of the cells, and intercellular spaces between them.
 
Further, Roux has examined the behaviour of the isolated cells of the Frog's egg in the morula stage. The cells were separated in a medium of albumen, or salt-solution, or a mixture of the two. They first approach and then flatten against one another (Cytarme), as do the blastomeres in the egg, completely or incompletely. The contact surface is generally symmetrical to the line joining the centres of mass of the two cells ; it may be concave towards either the small or the large cell. More than two cells may unite to form rows or heaps. The angles made by the surfaces of contact may be 120°, or have other values. Four surfaces may meet in one line ,- at other times the arrangement is tetrahedral. In a 1-25 Z solution of salt the cells are elongated, and united end to end in long branching strings. The pigment, diffused through the cell, later returns to its original position at the surface, or usually to the middle of the free surface of each.
 
The cells may also move over one another (Cytolisthcsis) by sliding or rotation, or both. Even two cells will glide on one another, as two soap-bubbles will not. In complexes two threesurface lines may unite to form one four-surface line, a behaviour the very opposite of that exhibited by soap-bubbles.
 
It appears, then, that in the living egg of the Frog (and other radial and bilateral types) there are factors which overcompensate, to use Roux’s expression, the purely physical factors by which the behaviour of the oil-drops is governed. These organic factors are that division is slow, and begins on the outside ; that the direction of division—determined by the yolk—is persistent; that the cell contents are neither perfectly fluid nor perfectly structureless ; that the cells being different, their surface tensions may be of dilferent magnitudes, and the whole system, therefore, not homogeneous; and that the cells possess a more or less solid rind or membrane, the rind which becomes wrinkled transversely to the furrow when the cell divides.
 
It would seem that this rind is an important factor, for if Roux’s experiments be repeated with drops of albumen suspended between xylol and oil of cloves, to which a little alcohol has been added, it will be seen that each drop gets a su1:erficial membrane, and that by these membranes adjacent drops adhere. In fact, such drops behave more like the cells of the egg‘ than do the oildrops. Thus, a small cell goes towards the inside, or the outside, according" to the way in which the division is made, and, after a horizontal division of four equal cells, the upper remain superimposed upon the lower.
 
At the same time, it is apparently because the cells have this surface film, which the oil-drops have not, that they are able to flatten against one another as soap-bubbles do; while, on the other hand, it is because the film is solid that the cells are unable to move upon one another and adopt the geometrical arrangement seen in systems of soap-bubbles.
 
There is still another kind of cell-movement to which brief reference must here be made, since it is found in one type of segmentation at least. In the segmenting eggs of some Platyhelmia (Triclads), Ascidians (Salps), Echinoderms (Asteroids), and Coelenterates (0ceam'a), the blastomeres have been seen to completely separate from one another, afterwards reuniting. Roux has observed a similar reunion of the artificially isolated cells of the Frog’s egg. This Cytotropism, as Roux calls it (Cytotaxis would be a preferable term), is noticed when the slide is kept perfectly horizontal and streaming movements of the medium (albumen) are rigidly excluded. The cells become rounded, and then approach one another in, more or less, a straight line, oscillating slightly backwards and forwards. The cells must not be too far apart, not further than a radius of small, or less of large cells. Groups of two or more cells behave in the same way.
 
The movement may be simply a surface-tension phenomenon, or, as Roux suggests, more complex, of the nature of a response to a mutual ehcmotactic stimulus.
 
These various kinds of cell-motion are also an important feature in such processes of differentiation as the union of cells to form muscles, tendon, epithelia, and so forth.
 
A review of all the facts thus leads us to conclude that while some of the phenomena of segmentation-—-the flattening of cells against one another, the pattern made by the cells in cleavage, especially of the spiral type—are largely referable to the action of the purely physical laws of surface tension, there are many cases, the radial and bilateral types, and the radial and bilateral periods of spirally segmenting eggs, in which the operation of these laws is restricted and confined by other causes. But in any case those laws can only co-operate with other factors, which are to be looked for in the rate and direction of division, and in the magnitude of the cells, factors which themselves are dependent on the relation between the cell and its nucleus.
 
Before concluding this section we have to call attention to some experiments which may possibly throw some light on an event of fairly frequent occurrence in ontogeny—the division of the nucleus without the division of the cell,‘ as in the formation of coenocytia such as striated muscle fibres and the trophoblast of the placenta ; or the fusion of distinct cells into a syncytium, as in the trophoblast again; or the secondary union of yolk-cells.
 
In the Alcyonaria the nucleus may divide three, four, or five times before the egg simultaneously breaks up into eight, sixteen, or thirty-two cells. See especially E. B. Wilson, ‘On the development of Renilla,’ Phil. Trans. Roy. Soc, clxxiv, 1883.
 
 
Driesch has observed that in the egg of Ea/Linus cell-division may be wholly or partially suppressed by pressure, and also by diluting the sea-water. Nuclear division continues (Fig. 33).
 
Morgan has found that the egg of another sea-urchin (Arbacia) will not segment in a 2 Z solution of salt in sea-water; on replacing the eggs in sea—water, however, the nucleus divides with great rapidity several times, and this is followed by celldivision. So Loch notices that the eggs when treated in this way, and brought back to their normal medium, divide simul taneously into four. The egg
 
es of the fish Cteuolabm.s (accord O“ o«° ing to the same author) behaves in a similar fashion when first f I deprived of, and then restored
 
,, 1, to oxygen. . FIG. 33.—Echinus: suppression of lmf’ again’ has Seen the re cell-division by 1)1'essure, I), and by union Of sister cells and nuclei
 
heat, a. Nuclear division continues. ' . ' . (After Driesch, 1893.) in the eggs of Avbacza leleased from pressure.
 
Three distinct agencics——mechanical pressure, increase of osmotic pressure, and decrease of osmotic prcssure—arc all capable of effecting this interesting change in the usual relations of cell and nucleus. We can only guess at the real cause, and surmise that it will be found in an alteration of internal and external surface tensions.
 
===Literature===
 
No'rE.—For a complete bibliography of segmentation the well-known textbooks of 0. Hertwig and Korsehelt and Heider must be consulted. The literature of ‘spiral’ segmentation is given by Robert (quoted below).
 
F. M. BALI-‘OUR. Comparative Embryology, London, 1885.
 
G. BERTHOLD. Studien fiber Protoplasmameehanik. VII. Theilungsrichtungen und Thcilungsfolge, Leipzig, 1886.
 
G. BORN. Ucber Druckversuche an Froscheiern, Anat. Anz. viii, 1893.
 
T. BOVERI. Die Entwiekelung von Asca;-is megalorephala mit beson— derer Riicksieht auf die Kernverhaltnisse, Fesfschr. Xupflkr, Jena, 1899.
 
E. G. CONKLIN. P1-otoplasniic movement as a factor of differentiation, Woods Hell Biol. Lech, 1898. II. I CELL-DIVISION 57
 
H. DRIESCH. Entwicklungsmechanische Studien, IV, Zeilschr. wiss. Zool. lv, 1893.
 
H. DRIESCH. Entwicklungsinechanische Studien, VIII, Mitt. Zool. Stat. Neapel, xi, 1895.
 
A. FISCIIEL. Zur Entwicklungsgeschichte der Echinodermen. I. Zur Mechanik der Zelltheilung. II. Versuche mit vitaler Fiirbung, Arch. Ent. Mech. xxii, 1906.
 
J. H. GEROULD. Studies on the Embryology of the Sipunculidae, Mark Anniversary Volume, New York, 1903. _
 
A. GRAF. Eine 1-iickgitngig gemachte Furchung, Zool. Anz. xvii, 1894.
 
C. HERBST. Ueber dais Auseinandergehen von Furchungs- und Gewebezellen in ka.lkfrciem Medium, Arch. Ent. M¢'c7z. ix, 1900.
 
O. HERTWIG. Die Zelle und die Gewebe, Jena, 1893.
 
O. HERTWIG. Ueber den Worth der ersten Furchungszellen fiir die Organbildung des Embryo, Arch. nu'l.-r. Anal. xlii, 1893.
 
O. HERTWIG. Ueber einige mu befruehteten Froschci durch Centrifugalkraft hervorgerufene Mcclnmomorphosen, S.-B. Kimigl. prcuss. All-ad. 11733., Berlin, 1897.
 
J. LOEB. Investigations in physiological morphology, Joum. Morph. vii, 1892.
 
J. LOEB. Untersuchungen fiber die physiologischen Wirkungen des Sauerstoffnmngels, I7l:2ger‘s Arch. lxii, 1896.
 
J. LOEB. Ueber Kerntheilung ohne Zellt-heilung, Arch. Ent. Mech. ii, 1896.
 
T. H. MORGAN. The action of salt solutions on the unfertilized and fertilized eggs of Arbacia and of other aniimtls, Arch. Ent. Mech. viii, 1899.
 
E. PFLi'IGER. Ueber die Einwirkung (ler Schwerkra.i't und andere Bedingungen nuf die Richtung der Zclltheilung, 1_’flc'«'ger‘s Arch. xxxiv, 1884.
 
J. PLATEAU. Statique des liquides, Paris, 1873.
 
A. RAUBER. Der karyokiuetische Process bei erhohtem und vermin(lertexn Atmosphiirendruck, Vcrs. Deutsch. Naimf. u. Aerzte, Magdaburg, 1884.
 
A. ROBERT. Rec-herchcs sur le développement des Troques, Arch. Zool. E.1'p. et G6». (3), x, 1902.
 
W. Roux. Ueber die Zeit der Bestimmung der Hauptrichtungen des Froschembryo, Leipzig, 1883, also Ges. Abh. 16.
 
W. Roux. Ueber den ‘Cytotropismus’ der Furchungszellen des Grasfrosches (Rmmfusca), Arch. Ent. Mech. i, 1894.
 
W. ROUX. Ueber die Selbstordnung (Cytotztxis) sich ‘beriihrendcr' Furchungszellen des Froscheies durch Zellonzusammenfiigung, Zellentrennung und Zellengleiten, Arch. Ent. Mech. iii, 1896.
 
W. Roux. Ueber die Bedeutung ‘geringei-' Verschiedenheiten der relativen Grfisse der Furchungszellen fur den Clim-akter des Furchungsschemas, Arch. Ent. Mech. iv, 1897. 58 CELL-DIVISION AND GROWTH II. 2
 
G. SMITH. Fauna und Flora. des Golfes von Neapel: Rhizocephala,
 
Berlin, 1906. O. ZUR STRASSEN. Embryonalentwicklung des Ascarisnmgalorephala,
 
Arch. Em‘. Jlfcah. iii, 1896. E. B. VVILSON. (_1leava.ge and mosaic work, An-7:. Ivlul. lilo:-72. iii, 1896. H. E. ZIEGLER. Ueber Furchung unter Pressung, Vcrh. Aunt. Gesell.
 
viii. 1894. H. E. ZIEGLER. Untersuehungen fiber die eisten Entwicklungsvor gétnge der Nematoden, Zeifschr. W1'.s-s. Zool. lx, 1895. H. E. ZIEGLER. Experimentelle Studien fiber die Zelltheilung, Arch. Elli. llferh. vii, 1898.
 
===2. Growth===
 
Following Davenport we define growth as increase in size or volume. Since, therefore, growth is increase in all three dimensions of space, it is most accurately measured not by increase in some one dimension—such as stature——but by increase of mass or weight.
 
Growth depends upon the intake of food and the absorption of water and exhibits itself in the form of increase in the amount of living matter or of secretions of watery or other substances, organic or inorganic, intra-cellular or extra-cellular, such as ehondrin, fat, muein, cellulose, calcium phosphate, and the like.
 
That growth depends———in later stages at least—upon the intake of food is obvious. That it is due to the absorption of water has been demonstrated effectively by Davenport for the tadpoles of Amphibia (zlmtlys/oma, Rana, I311/2)). The method employed was to weigh known numbers of the tadpoles at difl:'erent ages, desieeate and weigh again. The results of the investigation are shown in the accompanying figure (Fig. 34-), from which it will be seen that the percentage of water rises with remarkable rapidity———from 56% to 96% during the first fortnight after hatching. After that point the amount of water present slightly but steadily declines.
 
The same result is brought out by an analysis of the terminal buds and successive internodes of plants. It is found in I/ele7'oceutrou (Kraus) that the percentage of water rises rapidly from the terminal bud to the first internode, more slowly from the first to the second internode, and then remains constant.
 
 
It would thus appear that during the period of most rapid growth, growth is efifected by imbibition of water rather than by assimilation, since the weight of dry substance in the tadpole during this period does not increase at all.
 
In later development the proportion of water slowly falls. This may be seen not only in Davcnport’s table of the growth of Frogs but in the data furnished by Potts for the Chick and by
 
 
 
FIG. 34.— Curve showing change in percentage of water in Frog tadpoles from the first to the eighty-fourth day after hatching. Abscissae, days; ordinates, percentages. (After Davenport, from Korschelt and IIe1der.)
 
Fehling for the human embryo. These data. are given in the accompanying tables (Tables I, II). The percentage of water, at first high, slowly falls in both cases; conversely, the percentage of other substances increases.
 
‘ These results indicate that during later development growth is largely effected by excessive assimilation or by storing up formed substance’ (Davenport).
 
There are other external agencies by which growth may be affected in various ways—-such as heat, light, and atmospheric pressure. These will be discussed in another chapter. For the present let us confine our attention to certain features which are characteristic of growth in general, of the growth of the animal organism under normal conditions. These are the changes that
take place during growth in the rate of growth itself, in the variability of the organism and in the magnitude of the correlations between its various parts.
 
TABLE I
 
Showing the percentage of water in Chick embryos at various stages up to hatching. ( From Davenport, 1899 (2), after Potts.) ’1_‘he table also shows the hourly and daily percentage increments of weight.
 
Absolute Hourly Daily
 
H f . . P r entage
 
48 0-06 83 54 0-20 0-14 38-3 919-2 90 58 0-33 0-13 16-0 384-0 88 91 1-20 0-87 7-9 189-6 83 96 1-30 0-10 1-7 40-8 68 124 2-03 0-73 2 0 48-0 69 264 6-72 4-69 1 6 38-4 59 TABLE II
 
Showing the percentage of water in the Human embryo at various stages up to birth. (From Davenport, 1899 (2), after Fehling.) The table also shows the weekly percentage increments of weight.
 
Age in Absolute weight \Vec-kly per- Percentage
 
weeks. in grammes. Increase‘ ccntnge increment. of water.
 
6 0-975 97-5 17 36-5 35-525 331-2 91-8 22 100-0 63 5 34-8 92-0 24 242-0 142-0 71-0 89-9 26 569-0 327-0 67 6 86-4 30 924-0 355-0 15-6 83-7 35 928-0 4 0 0-1 82 9 39 1640-0 712-0 19-2 74-2
 
VVe follow Minot and Preyer in measuring the rate of growth by the percentage increments of weight (or of other measurements where weight is not available) during a given interval of time ; that is to say, by expressing the increase in weight during a given period as a. percentage of the weight at the beginning (or end) of that period. The change of rate, if any, is found by taking such percentage increments for successive equal increments of time.
 
As a first example let us consider the data furnished by Minot II. 9. GROWTH 61
 
himself for the rate of growth, after birth, of guinea.-pigs (Table III, Fig. 35).
 
TABLE III
 
Showing the change of rate of growth in male and female Guinea.-pigs. as measured by daily percentage increments of weight. (From Minot, 1891.)
 
Average daily per cent. Average daily per cent.
 
Ago in Age in
 
d“Y‘- Ma1§§°'°m§'§§$1cs. “‘°““‘s- Ma1§§.°r°m°§§:.a1..s. 1-3 0.0 2.1 8 0.05 0.2 4-6 5.6 5.5 9 0.3 0.2 7-9 5.5 5.4 10 0.1 0.1
 
10-12 4.7 4.7 11 0.04 0.1
 
13-15 5.0 5.0 12 0.1 0.05
 
16-18 4.1 4.3 13 -0.2 0.3
 
19-21 3.9 3.5 14 0.5 -0.03
 
22-24 3.1 1.7 15 0.2 0.00
 
25-27 2.3 1.9 16 0.07 0.2
 
23-30 2.3 2-6 17 -0.1 -0.02
 
31-33 1.9 1.3 13 -0.05 -0.2
 
34-36 1.7 1-6 19-21 0.006 -0.1
 
37-39 1.9 1.3 22-24 0.02 -0.05
 
40-50 1.2 1.1
 
55-65 1.3 1.3
 
70-80 1.2 0.3
 
35-95 0.9 0.9
 
100-110 0.7 0.3 115-125 0.6 0.5 130-140 0.1 0.2 145-155 0 4 -0.03 160-170 0.3 0.5 175-135 0.2 0.2 190-200 0.2 0.2 205-215 0.4 0.3
 
 
15Il| I7 D E II N 75 I50
 
FIG. 3-'3.—Curve showing the daily percentage increments in weight of female Guine-.1-pigs. (From Mxnot, 1907 )
 
An inspection of tlie accompanying table and figure in which Minot’s results are reproduced will show at once that there is in both sexes, almost from the moment of birth, a. decline in the growth-rate. The decline is not, however, uniform. The rate falls rapidly between about the fifth day (when it is from 5% to 6%) and the fiftieth, from the fiftieth day onwards more slowly, becoming eventually very small, zero or even negative. The younger the animal, therefore, the faster it grows; the more developed it is the more slowly it grows. The rate of growth in fact varies inversely with the degree of difierentiation. A mammal, therefore, which is born in a less developed condition than is the guinea-pig ought to grow at first more rapidly still. The rabbit is such an animal, and Minot has been able to show that on the fourth day after birth the young rabbit adds 17 % to its weight. The curve also shows the same rapid decline in the growth-rate as was observed in the guinea-pig, followed by a period of gentle decrease.
 
Accurate observations on the prenatal rate of growth of these two mammals are lacking, but Henscn’s few observations (quoted by Preyer) on the weight of guinea-pig embryos show that the daily percentage increase descends from 220% on the twenty-first day to 116% on the twenty-ninth day, to 33% on the forty-third (lay, and again to 6% on the sixty-fourth day, that is just after birth, the moment at which Minot’s observations begin. Again, Minot has found, as a result of the investigation of the weight
 
TABLE IV
 
Showing the decrease in the rate of growth of the Human embryo before birth. Percentage increments calculated from the figures given by
 
Hecker, Toldt, and Hennig. (From Preyer.)
 
Average monthly percentage increments of Month. Weight. Length.
 
(Hecken) (Toldt.) (Hennig.) 1 _ _ __ 2 — 133-3 433 0 3 — 100-0 110-0 4 418-2 71-4 92-8 5 398-2 66-7 69-8 6 123-2 50-0 28-2 7 92-2 16-7 14-2 8 28 8 14-3 11-9 9 25-6 12-5 6-3 10 — 11-1 4 3 II. 2 GROWTH 63
 
of spirit specimens of rabbit embryos that the mean daily percentage increment is 704 between the ninth and fifteenth days, but between the fifteenth and twentieth days only 212.
 
The postnatal decline in the growth-rate is therefore only a continuation of a process which has been going on for some time, perhaps from the first moment at which growth began.
 
The human being forms no exception to this rule. Data of the growth of the human embryo before birth are somewhat meagre, but an inspection of the tables will show that whatever the discrepancies may be between the results obtained by Fehlin g (Table II) and Hecker (Table IV), they agree in this, that the growth-rate falls with great rapidity between the fourth and the sixth months,thereafter more slowly till the end of pregnancy. This is graphically represented in the curve (Fig. 36). It will be observed from the table (Table IV) that the rate of increase of stature also declines, but less abruptly. This is a poi11t to which we shall return. For the study of the postnatal growth of man very numerous data have FIG’ 36' —' Curve Sh°‘”il‘<‘=’
 
_ monthly prenatal percentage 1nbecn collected byvarious observers. cmments in Man. (From Minot, Measurements of the bed y weight 1907-) have been made on Belgians by Quetelet, on Boston school children by Bowditch, on the school children of Worcester, Mass., and Oakland, Mass., by Boas, and on English of the artisan and the well-to—do classes by Roberts. It is unnecessary to reproduce all these data here, for they all show the same decline in the growth Monnvso-zaascrogm 64 CELL-DIVISION AND GROWTH II. 2
 
rate, but Quetelet’s measurements for males, being the completest series, are given in the accompanying figure (Fig. 37). The figure shows that at the end of the first year after birth the per.
 
 
 
FIG. 37.——Curve showing the yearly percentage increments in weight of Boys. (From Minot, 1907.)
 
centage increment is as high as 200% (or nearly), but that then this increment drops to just over 20% at the end of the second year. From this point the decline is slow but sure, until at the thirtieth year the annual percentage increase is only 0-1 %. The change of rate of growth in females is practically the same as in males. The monthly percentage increment immediately before birth is about 20% according to Miihlmann’ s curve (Fig. 36) ; this represents an annual percentage increment of, say, 250 %, and the annual increase at the end of the first year is about 200 %. The postnatal deerease of growth is, therefore, as in other mammals, a continuation of the prenatal change. Further, there are two points at which the rate diminishes with great rapidity-—between the fourth and sixth months of pregnancy and between the first and second years after birth. It would be of the greatest interest to discover the causes of these sudden decreases. Elsewhere the II. 2 GROWTH 65
 
diminution is gradual. A point of importance is that in both years there is a slight temporary rise in the growth-rate about the time of puberty (see the curve, Fig. 37'). This has been noticed by all observers, but the actual time of its occurrence ditfers in diEercnt cases ; the rise is invariably earlier in females than in males. A comparison of the growth of the three mammals considered is interesting. A Guinea-pig reaches 775 grammes in 43.2 days. A Rabbit ,, 2,500 ,, 395 ,, A Man ,, 63,000 ,, 9,428 ,, or the average daily increment is for a Guinea-pig L82 grammes. Rabbit 6-30 ,, Man 669 ,,
 
Hence ‘men are larger than rabbits because they grow longer, but rabbits are larger than guinea—pigs because they grow faster’. Minot, however, distinguishes between the ‘rapidity’ of growth, the average actual increment, and the ‘ rate’ of growth, the percentage increment. The average percentage increments for these mammals are
 
Guinea-pig 0-4-7
 
Rabbit 0-50
 
Man 0-02
 
The rate is, therefore, much slower in man than in the otln r two. These percentages Minot calls the coefficients of growth. Together with the duration of growth they determine the ultimate size of the organisms.
 
The progressive loss of growth-power Minot speaks of as ‘senescence’, and compares to the loss of the power of celldivision in the ‘senile decay’ of Protozoa. The same author has also brought forward evidence to show that during differentiation there is an increase in the amount of cytoplasm in the cell, a decrease in the size of the nucleus, and a decrease in the ‘mitotic index’, that is in the proportion, in any tissue, of dividing cells. During segmentation, of course, the reverse of this is taking place, since cell-division is rapid and the protoplasm per cell is being constantly diminished until a fixed ratio between nucleus and cytoplasm is reached (Boveri) (see below, p. 268). Minot suggests that ‘ senescence ’ and differentiation alike depend on an increase in the protoplasm.
 
 
FIG. 38.— Curves showing the change with age of the rate of growth In the larva of the sen.-urchin Strongylocentrotus (from Vernon's data), and the pond-snail Limnaea (from Semper's figures). The nbscissae are days, the ordinates percentage increments.
 
 
The decline of the growth-rate may also be seen in Pott's weighings of the Chick embryo before hatching (Table I) and Minot’ s Weights of young chickens. It appears from these that the daily percentage increment is 919 % at the beginning of the third day of incubation, 189% at the end of the fourth day; at this point there is a sudden drop to 40 Z, which is still the rate of growth after eleven days of incubation; eight days after hatching the ra.te is 9% in the male, not quite 9% in the female, and then comes a period of more or less gradual decline, until when the chicken is 342 days old it is able to add less than 0-5 % to its weight per diem.
 
Sempei-’s observations on the pond-snail, Limizaea, and Vernon’s on the sea-urchin, St1'0ngyloce7m'ot1m, are other examples which
 
 
 
FIG. 39.—Dai1y percentage increments of weight in tadpoles: the continuous line (a) gives the whole weight, the broken line (b) the dry weight. (After Davenport, 1899.)
 
may be mentioned. The results of these autl101's are shown in the accompanying charts (Fig. 38). Their measurements are of lengths, not of weights.
 
So far we have found no exception to the law of the decline in the rate of growth as development proceeds. Davenport's measurements of tadpoles will not, however, conform to the generalization. As the figure shows (Fig. 39), the daily percentage increments, whether of the whole weight or of the weight of dry substance only, first rise abruptly, then descend and then rise again. An explanation of this anomaly may possibly be found in the fact that Davenport/s measurements are 68 CELL-DIVISION AND GROWTH II. 2
 
taken during that early period when growth is due in the main to absorption of water, the other measurements (as may be seen from Tables I, II) during the later period when the percentage of water has already begun to decline and growth is effected by other means. '
 
It is_, of course, a commonplace of embryology that the growth of all the organs of the body does not occur at the same rate.
 
 
 
FIG. 40. — Curves showing the alteration during the first twenty years
 
of life of the rate of growth as measured by weight, stature, and chestgirth in the human being (males). (Constructed from Quetelc-t’s data.) The abscissae are years, the ordinates percentage increments. (The percentage increment of weight for the first year could not be included in the figure. It is given in Fig. 37.) There are nevertheless few cases in which the exact difference in rate has been ascertained. From those few cases, however, it appears that the individual parts, though they do not grow with equal rapidity, still obey the same law as the whole.
 
Thus human stature exhibits the same loss of growth-power as is shown by the weight of the whole body, with this  difference, however, that the rate is not so high in early stages, the descent in later stages less abrupt. This will be seen in Table IV, in which such figures as are obtainable for the prenatal growth-rate are given, and in Fig. 40, in which the curve of change of growth-rate in human stature has been constructed from Q.uetelet’s data (male Belgians). The percentage increment in the first year is only 39-6 as against 190-3 for weight, in the second year 13-3 us against 22-2 for weight. Thenceforward the rate slowly declines, until at the fortieth year it is zero, and after the fiftieth year increasingly negative. The rate of increase of stature is always slightly less than that of weight. Q.uetelet’s figures do not show the rise in rate about the time of puberty, but this phenomenon is apparent in the data furnished by Bowditch, Boas, and Roberts (see Fig. 4-2). The change in the growth-rate is practically the same in women as in men. As with weight, the rise of rate at the time of puberty is earlier.
 
 
 
 
 
FIG. 41. — Curves showing the alteration during the first twenty years of life of the rate of growth of stature, length of head, length of vertebral column, and length of leg in the human being (males). (Constructed from Quetelet’s data.) Ordinates, percentage increments; abseissae, years.
 
 
 
The decline in the growth-rate of chest-girth is shown in the same figure (Fig. 40). It will be noticed at once that in this case the drop in the first year is very great indeed, from nearly 50 % to nearly 5 %, and that the rate is only diminished by another 24 ‘Z in the next nineteen years. The weight will depend upon the volume and the volume on both stature and girth; in. fact a rough weight-curve might be constructed from the measurements of stature and girth. It is evident that the sudden loss in the rate of total growth after the first year is due to the very rapid decrease in the percentage increment of girth.
 
It maybe mentioned that other measurements of girth—girth by the sternum, the waist, the hips, the neck, the biceps, the thigh——show the same exceedingly abrupt decrease, almost to the minimum rate, between the first and second years.
 
In other cases——distance between the eyes, width of mouth, length of hand, length of foot, arm-length, leg-length, length and breadth of head, distance from the crown of the head to the first vertebra, length of the vertebral column—the change is more gradual ,- the rate of change, however, diifers in different cases. As an instance of this let us consider the measurements—— from the crown to the first vertebra, the length of the vertebral column, and the leg-length——whieh together make up the total stature. The growth-curves of these three and of the whole stature are presented in the figure (Fig. 41), from which it will be seen that the growth of the leg is faster than that of the vertebral column (until the eighteenth year), and this than that of the head. Increase in stature takes place at nearly the same rate as that of the vertebral column , but is on the whole a little faster.
 
There are few cases—besides man—in which we possess information as to the growth of the parts. In the sea-urchin, S57'07I_q,I/[Odell/7'0tl(8, Vernon has shown that the growth-rate of the oral and aboral arms of the Pluteus diminishes rapidly from
the third to the fifth days, more slowly from the fifth to the eighth days. After this the rate becomes negative, as the skeleton of the Pluteus is used up by the developing urchin. The curves of change of rate of growth-—as constructed from Vernon’s figures—-are shown in the chart (Fig. 38).
 
In Oarcirzm mamas a gradual decrease in the growth-rate of the frontal breadth can be ascertained from Weldon’s data.
 
We have next to consider another feature of growth, the alteration of variability. The facts at our command are derived from a study of Echinoid larvae (Vernon), Duck embryos (Fisehcl), Guinea-pigs (Minot), the Periwinkle (Bumpus), the Crab, C'arcz'mts (Weldon), and the human being (Bowditch, Pearson, Robeits, and Boas). Vernon has shown that in the Pluteus of Stronyy/ocentrolus the variability of the body-length increases regularly up to the fifth day, and then decreases regularly again to the sixteenth day. So Fischel’s measurements of Duck embryos seem to establish a greater variability in younger than in older stages. This is true of the whole length, the head (as far back as the first somitc), the hand and trunk together, and the total length exclusive of the primitive streak. The data are, however, too few to be treated statistically; the variability can only be roughly estimated from the extent of the limits within which the part varies at each stage.
 
Minot, who expresses the variability of guinea-pigs by the difference between the mean weight and the mean weight of the individuals above, and of those below, the mean, likewise finds that the range in variation diminishes with age, and further that, in the case of the males, there is a pcriod—-from about the fourth to the ninth months—when the variability is very much less than at any other time. No such sudden fall is observed in the female, only a steady diminution.
 
A more satisfactory calculation of the altera.tion of variability may be made from the measurements taken by Bumpus of the ‘ventricosity’ (ratio of breadth to length) of the shell of the Periwinkle, Lit/orina lit/area. The series of observations is very large, and includes both British and American forms. In the accompanying table (Table V) the coeflicients of variability (the standard deviation expressed as a percentage of the mean) are given for each age, as determined by length of shell, for the English and American periwinkles separately and also for the complete series. It will be seen that the variability increases slightly and then diminishes again. This is the case also in the American examples, where the fall at the end of growth is greater still, but in the British specimens there is only a slight fall, at 20-21 mm., followed by a considerable rise. The possible significance of this diiference in the behaviour of the same species on the two sides of the Atlantic we shall discuss
 
in a moment. TABLE V Showing the alteration in the variability of the ventrieosity of the shell of the Periwinkle (Liltorina littorea.) during growth.
 
Coefficient of variability (E; x 100).
 
Length in mm. British. American. All. -15 2-77 3-27 3-25
 
16-17 2-94 3-41 3-34 18-19 3-02 3-39 3-35 20-21 2-93 3-03 3-13
 
22 3-25 2-84 3-02
 
In the meantime let us consider another case, the Crab, Cr/rcimc.v mocnas, the variability of the frontal breadth of which was examined by the late Professor VVelden. Weldon found that the variability, as measured by the quartile error , first increased and then suddenly diminished with age (as determined by carapaee length). If the variability is measured by the coefficient of variability (easily calculated from VVel(lon’s data) the result is the same. This will appear from the table
 
(Table VI). TABLE VI
 
Showing the change in the variability of the frontal breaulth with age in Curr-inns momms. (After Weldon.)
 
Cara ace Len th Q. 0'
 
E, ,,,m_ 3 51- x 100. 7-5 9-42 1-64 8-5 9-83 1-76 9-5 9-51 1-73 10-5 9-58 1-78 11-5 10-25 1-93 12-5 10-79 2-06 13-5 10-09 1-95
 
For the calculation of the variability at different ages in man data have been provided by Roberts, Bowditch, and Boas. Some II. 2 GROWTH 73
 
of these results are collected in the following table (Table VII), from which it may be gathered that the variability diminishes at first, then rises until it attains a maximum at about the time of puberty, and then diminishes again, reaching finally a value which is lower than the original. The values for the coeflicient obtained by the diifercnt investigators are fairly similar, and agree very well with those first given by Pearson (for male new—born infants, weight 15-66, stature 6-50; for male adults, weight 10-83, stature 3-66). It may be seen from the values for the newborn that the variability has already undergone adiminution before the age at which the other observations begin.
 
TABLE VII
 
Showing the change in the value of the coefiieient of variability in the male Human being during growth.
 
Coeffioieiit of variability (1% X 100).
 
Vtleight. Stature. Years. Boston Worcester, English Boston Toronto \Vorecstor, (Bowditch). Mass. Artisans (Bowditch \. (Boas). Mass. (Boas). (Roberts). (Boas) 4 14-00 5 11-56 11-48 4-76 4-82 6 10-28 12-04 10-08 4-60 4-34 5-40 7 11-08 11-87 10-29 4-42 4-35 4-24 8 9-92 11-83 10-78 4-49 4-58 4-32 9 11-04 12-29 10-85 4-40 4-41 4-30 10 11-60 12-92 11-06 4-55 4-68 4-44 11 11-76 14-45 11-90 4-70 4-53 4-51 12 13-72 15-56 11-48 4-90 4-85 4-49 13 13-60 18-07 11-76 5-47 5-36 5-21 14 16-80 16-80 12-74 5-79 5-64 5-43 15 15-32 18-28 14-00 5-57 5-71 5-19 16 13-28 13-95 12-95 4-50 3-92 17 12-96 11-23 11-55 4-55 3-32 18 10-40 12-18 3 69 19 10-29 20 9-03 10-50 10-92 12-04
 
Further, the variability does not merely diminish as the animal grows older. Its diminution accompanies the diminution in the rate of growth, and when—as at the time of puberty in man—that rate increases, the variability increases too.
 
The variability of such parts as have been examined for the
purpose alters in the same way as that of the whole body. Besides weight and stature Boas has recorded measurements of height sitting, head-length, head-width, length of fore-arm, and hand-width.
 
Though the evidence, it must be admitted, is scanty, it is none the less a remarkable fact that in all the cases we have examined the variability, whether of the whole organism or of its parts, decreases with the decrease in the rate of growth. We seem to be in the presence of a phenomenon of general occurrence, though what the significance of the phenomenon is is not at present clear.
 
As is well known, Weldon has argued that the decline in the variability of the older crabs is due to a selective death-rate, an argument which is supported by the same author’s observations on the snail Clausilia, since in this form the variability of the adult was found to be the same as the variability of the same individuals when young, but less than the variability of the general population of young. It is possible that the marked decrease in the variability of the American as compared with the British periwinkles may also be attributable to the same cause, since this animal has only recently been introduced into America, and may, therefore, be subjected to a more severe struggle for existence in its new environment.
 
It is doubtful, however, whether this explanation will fit all cases.
 
Vernon has suggested that at periods of rapid growth the effect produced upon the organism by a change in its environment must be much greater than at other times, and, since he has further shown that one of the effects of an adverse change of circumstances is an increased variability, he argues that an increase in variability would naturally accompany a high growthrate.
 
Lastly, Boas points out that the rate of growth is itself a variable magnitude, and this ‘ variation in period’ may, with other causes, be a factor in producing the actual variation at each stage. Should that be so, the variability would necessarily increase and diminish with an increasing and diminishing growthrate, since those that are above the mean would tend to remove themselves further from the mean than those that are below could approach it, and the more so the faster they were growing, and conversely.
 
We have finally to consider very briefly what little is known of the alteration with growth in the value of the correlation between various organs. Such data as we have indicate that, like variability, this value rises and falls with the growth-rate.
 
Boas has ascertained the correlation coefficicnt (p) in man between weight, stature, height sitting, length and width of head at diiferent ages. Some of these results are tabulated below (Table VIII) ; from this table it will be evident that the value of p decreases, increases, and decreases again. The values are for girls, and the period of increase is earlier than that found for boys. In the chart (Fig. 42) are given the successive values of growth-rate (stature), variability (height), and correlation coefficient (height sitting and head-length) for boys; the three magnitudes rise at about the time of puberty, and subsequently decline together. Boas urges that if the actual variability is in part the efi’cct of variation in period, this eifect will be greater during periods of rapid development. It follows from this that if the various organs of the body are equally affected by a change in the growth-rate, correlations would be closer during periods of rapid growth than at other periods.
 
TABLE VIII
 
Values of the correlation coefficient, p, during growth for four diflbrent correlations. Girls, Worcester, Mass. (Boas).
 
Years. Stature and Stature and Stature and Stature and Weight. Height sitting. Length of Head. Width of Head. 7 -73 -74 -30 -21 8 -76 -79 -36 -15 9 -80 -82 -35 -16 10 ~83 -83 -37 -16 ll -81 -84 -37 -25 12 -77 -82 -38 -27 13 -73 -83 .38 -37 14 -67 -82 -30 -25 15 -6!’) -79 -26 -22 16 -60 -74 -25 -10
 
 
It will be noticed that the value of p for the different organs is different, being greater between axial organs—stature and height sitting, stature and length of head-—than between longitudinal and transverse parts, such as stature and width of head. The correlation between stature and weight is liigli.
 
To whatever cause it may be due this diminution of correlation with age is of the greatest interest, since it points to an increasing power of self-difierentiation in the parts of the body. From other sources also there is evidence of a. progressive loss of totipotentiality of the parts, of an inereasing- independence of the parts, of a tendency to be increasingly governed in their development by factors that reside wholly within themselves. But this evidence must be discussed elsewhere.
 
 
 
 
FIG.42.- Figure to show how the rate of growth (percentage increments of stature), the variability (of stature) and the correlation coefficient (between height sitting and length of head) rise together at the time of puberty in man and then fall together. (Constructed from the tables of Boas.)
 
 
 
===Literature===
 
F. BOAS. The growth of’ Toronto children, U.S.A. Report of the Commissioner of Ed'ucatz'on, ii, 1897.
 
F. BoAs and C. WISSLER. Statistics of growth, Unitecl Slates Education Commission, i, 1904.
 
H. P. BOWDITCII. The growth of children, Massachusetts Sfale Board of Iloalih, 1877.
 
II. C. BUMPUS. The variations and mutations of the introduced Littorina., Zool. Bull. i, 1898.
 
C. B. DAVENPORT. The role of water in growth, Proc. Boslou Soc. Nat. Hist. xxviii, 1899 (1).
 
C. B. DAVENPORT. Experimental morphology, New York, 1899 (2).
 
A. FISCIIEL. Ueber Varhibilitiit und VVa.ehstl1um des embryonalen Kéirpers, lllurph. Jahrb. xxiv, 1896.
 
G. KRAUS. Ueber die Wa.sse1-vertheilung in der Pflmize, Fests-rhr. Fem‘ Iuuulertjiilu-. Best. Natmf. Ges., Halle, 1879.
 
C. S. MINOT. Seneseence and rejuvenation, Jouru. I’h_2/s. xii, 189].
 
C. S. MINOT. The problem of age, growth and death, Pop. Sci. Jllonthlg/, 1907.
 
K. PEARSON. Data. for the 1)l‘Ol)1el1l of evolution in man. Ill. On the magnitude of certain coeflicients of correlation in mam. 1’rac. Roy. Soc. lxvi, 1900.
 
W. PREYER. Spezielle Physiologie (les Embryo. VIII. Dns embryona.le Wachsthum. Leipzig, 1885.
 
A. QUETELET. Anthropometric, Bruxelles, 1870.
 
C. ROBERTS. Manual of a.nthropomet1'y, London, 1878.
 
K. SEMPER. Animal Life, 5th ed., London, 1906.
 
H. M. VERNON. '1‘he effect of environment on the development of Echinoderm larvae : an experimental enquiry into the causes of variation, Phil. Trams. Roy. Soc. elxxxvi, B, 1895.
 
II. M. VERNON. Variation, London, 1898.
 
W. F. R. WELDON. An attempt to measure the death-ra.te due to the selective destruction of (larcinus mamas with respect to a, pu.rticul:|.r dimension, 1’roc. Roy. Soc. lvii, 1894-5.
 
W. I‘‘. R. WELDON. A first study of na.tura,l selection in (L'luu.x-iliu Imm'nalu, Biometrilca, i, 1901-2.
 
==Appendix A==
 
FURTHER REMARKS ON  RELATION BETWEEN THE
SYMMETRY OF THE EGG, THE SYMMETRY OF SEG
MENTATION, AND THE SYMMETRY OF THE EMBRYO IN
THE FROG.
 
IN the measurements, referred to above (pp. 165-8), of the
angles between the plane of symmetry of the egg (as determined by
the position of the grey crescent), the first furrow and the sagittal
plane of the embryo, it was found (1) that there was a certain
tendency for the first furrow and the sagittal plane to coincide,
since in a. large number of cases small angles preponderated over
large ones, the standard deviation of this angle from the mean
(which was practically = 0°) being a- = 40-39° i-65 ; (2) that
there was a much greater tendency for the plane of symmetry
and the sagittal plane to coincide, the standard deviation of the
angle between these two planes being o'=29-75° _-J; -63 ; (3) that
the first furrow tended either to coincide with or to lie at right
angles to the plane of symmetry, the standard deviation about 0°
being 18-70° i -60, that about 90° being 23-29° j-_ -86, the value
of 0' for all the observations being 47-90° 1- 1-19. The
correlation between the first furrow and the sagittal plane was
found to be p=-138i -031, that between the plane of symmetry
 
and the sagittal plane p=-372i -025, that between the plane of
symmetry and the first furrow p=-O87 i -032.
 
These results may be tabulated as follows :
rr :0
40-39° + -65. -138 i -031.
 
tal Plane.
 
Plane of Symmetry and
Sagittal Plane. l 2975 -t '63’
 
Plane of Symmetry and
First Furrow.
 
First Furrow and Sagit- }
 
.372 i .025.
 
} 47.9oi1.19. .os7¢_.o32.
 
Full details of these results will be found in a paper in
Biometrika V. 1906.
 
For the purpose of making these measurements the eggs were
placed in rows parallel to the [mat]; of glass slides, and the
angles measured between the various planes and lines ruled
across the slide. Such eggs compress one another by their jelly coats; further, the eggs taken ‘from the uterus were placed
haphazard on the slides with the axis making any direction with
the vertical. The egg takes about half-an-hour to turn into its
normal position with the axis vertical, and during this interval
gravity may possibly act upon the yolk and protoplasm, of
different specific gravities, and impress a plane of bilateral
gravitation symmetry upon the egg, as occurs when the egg is
permanently inverted (see above, pp. 82-87). This obliquity of
the axis may possibly afiect the relations between the planes,
and the mutual compression may also be a disturbing factor,
since it is known that in compressed eggs the nuclear spindle is
perpendicular to the direction of the pressure (pp. 34-36).
 
These angles have therefore now been measured under four
different conditions:
 
(a) The eggs are close to one another in the rows and the axis is
horizontal.‘ (Since the rows are parallel to the length of the slide
the pressure, if any, must be in the same direction, while the
surfaces of compression or contact are across the slide. The eggs
were always so placed that the vegetative poles faced in one
direction and the planes of ‘ gravitation symmetry ’ were at right
angles to the length of the slide. This holds good of all the
following experiments.)
 
(/3) The eggs close, but the. axis vertical with the white pole
below. In these there can be no gravitation plane of symmetry.
 
(y) The eggs spaced, but the axis horizontal. In these the
jellies do not touch.
 
(6) The eggs spaced and the axis vertical. In these, therefore,
both the supposedly disturbing factors are removed. The results
are given in the following table :—
 
A B C
 
First Furrow and Plane of Symmetry Plane of Symmetry
Sagittal Plane. and Sagittal Plane. and First Furrow.
 
(.7) .7 = 38-42 g._ -70. .7 = 31-86: -56. .7 = 41-591-_-84.
,7 = -201;-028. ,7 = -263;:-_-027. ,7 -= -118;-029.
(.9) .7 = 33-443-_-56. .7 = 30-17:51. .7 = 39-7l_-1;-61.
,7 = 3523-021. .7 = .27si.o22. ,7 = .o23¢.o24.
(-,) .7 —_- 33-49;:-_-96. .7 = 27-53¢-84. .7 = 36-60: 1-108.
,. = .292:-039. —_— -399:-036. p = .075:-043.
(a) .7 = 31.45133. .7 —_— 26-80¢-82. .7 = 34-46:1-065.
,7 —_- -364:-033. ,7 = -451 1.035. ,7 -_- -186;-043.
 
It is evident from this that gravity and ‘ mutual compression ’
(as I will for the moment term it, though it is doubtful whether
the pressure has anything at all to do with the result) do affect the
 
magnitude of the angles between these three planes, for in each case the standard deviation falls, while the correlation coeflicient
rises, when they are both removed. It will be observed that,
while gravitation (y) has less eflect than compression (3) upon the
angles B and C, the reverse is the case with the angle A. We
may be able to find a reason for this later on.
 
There is one point worth noticing. It is quite clear that
gravity is not indispensable for the development of a grey
crescent and plane of symmetry, though it is true that the position
of this plane may be aifected by gravity even in the short interval
that elapses before the egg turns over.
 
The values for the compressed eggs with horizontal axes (or)
compare fairly well with those previously obtained, except in the
case of the plane of symmetry and the first furrow. In the
former series the latter tended either to coincide with or to lie
at right angles to the former. In the present series this is not
the case. This diflerence is probably to be attributed to the
fact that many of the eggs in the first series must have been
placed on the slide with the white pole upwards: possibly also
the ‘ compression ’ was greater then than now.
 
It is fortunate that the same data enable us to study exactly
the relation between the first furrow and the plane of symmetry
on the one hand, and the direction of ‘compression’ and of the
gravitation symmetry plane on the other. It must be remembered that these two are at right angles to one another.
 
Consider first the first furrow.
 
(a) When the eggs are close but the axis horizontal the first
furrow tends to lie at right angles to the slide, that is, in the
direction of compression, but at right angles to the gravitation
symmetry plane. (a-=38-16 i -69.)
 
(fl) When the eggs are close but the axis vertical this tendency
is not quite so marked. (a'=46-67 i -7' 1.)
 
(y) When the eggs are spaced and the axis horizontal it is
still there, but slight. (o-=49-32 ;l-_ 1-40.)
 
(6) When the eggs are spaced and the axis vertical the
direction of the first furrow is random. (zr=52-76¢ 1-17.)
 
VVe may conclude, therefore, that the first furrow tends to lie
in the direction of the ‘ compression’ and at right angles to the
plane of gravitation symmetry. The latter tendency, we know,
exists in forcibly inverted eggs, together with a tendency to lie
in the plane of symmetry and at 45° to it (above, p. 84).
Pressure experiments alo show that division is in the direction
of pressure (p. 34 sqq.).
 
The direction taken up by the plane of symmetry under these
different circumstances is-quite distinct from that of the first
furrow. It appears to be determined in the first instance by gravitation, as it usually lies in the gravitation symmetry plane.
It is not, however, only so determined, for if the eggs (compressed
and with axi horizontal) be allowed to develop in the light the
plane of symmetry lies either in the gravitation symmetry plane,
or in the direction of the incident light (parallel to the length of
the slide in the experiment , while in the dark it lies only across
the slide. That this secon effect is due to the light and not to
the pressure is shown by the fact that it occurs when the eggs
are spaced, and that it may be made to vary in position by
varying the position of the slide with regard to the light.
Light, therefore (ordinary daylight), as well as gravity, can help
to determine the -position of the plane of symmetry, and when
the latter is excluded it appears that this plane is placed either
in or at right angles to the source of light.
 
Light appears to exert no effect u on the first furrow.
 
It is now intelligible why, when 1 these factors are operative,
the relation between the first furrow and the planes of symmetry
of egg and embryo should be disturbed, since, in the conditions
of the experiment, those factors which determine the position of
the former are at right angles to those on which the direction of
the latter depends.
 
It still remains for us to inquire into the internal causes of
the direction of these planes in the egg. Roux, as has been
pointed out, has asserted that the grey crescent appears on the
opposite side of the egg to that on which the spermatozoon has
entered (pp. 80, 165), and further that the point of entry of the
sperm also determines the meridian of the first furrow, since this
either includes the sperm-path, or is parallel to it, or, when it is
crooked, includes or is parallel to the inner portion or ‘ copulation ’
path, which is taken to represent the line of approximation of
the two pronuclei; the outer part being simply the ‘ penetration’
path. Roux also arbitrarily selected a fertilization meridian
(meridian of the sperm-entry), and showed that this became
the ventral side (opposite the grey crescent) later on, as well as
the ineridian of the first furrow (p. 248).
 
I have been able to accurately investigate—by means of
sections-—the relation between the fertilization meridian, first
furrow, and sperm-path in a number of eggs in which the
direction of the symmetry plane had been previously determined,
and the results of the measurements of these angles are given
here. The eggs fall into two series, those which were compressed
and had their axes horizontal (a), and those which were spaced
and had their axes vertical, the white pole being below (6). In
(a) the gravitation symmetry plane and the direction of compression were at right angles to one another, as before.
 
8 a
Meridian of sperm entry a- = 21-02° 1-_ 1-63. o- = 31-04°: 1-34.
and first furrow. p = -435 3 -074. 9 =-613 i -038.
 
Meridian of sperm entry 0' = 25-67° i 1-35. 0 = 41-01° 3-_ 1-78.
and symmetry plane. p = -302 i -083. / P = -006 1 -061.
 
SP§;§§f:;l3(g;§‘}ff1,§§;“ } . .. .—. 17.94° : 1.15. o‘ = 21.47° 1 -93.
 
From this it is clear that there is a very close relation indeed
between the point of entry of the spermatozoon and the direction
of the first furrow, especially when the disturbing efiects of pressure and gravity are removed. There is, however, little relation
between the sperm meridian and the plane of symmetry even
under the most favourable circumstances, and when the conditiofis are not favourable the correlation is negligible. There is
however (in the 6 series) a considerable correlation (p = -479 i '070)
between the sperm-pat/l and the plane of symmetry. It should
be remembered, however, that all these eggs were exposed to the
light. From what we know of the eifect of this agent upon the
direction of the symmetry plane, it would not perhaps be too
hold a hazard to surmise that in darkness there would be a
correlation between the sperm entrance and the plane of symmetr .
 
Eiien after the removal of this disturbance there remain
factors which interfere with the completeness of the correlation
between these planes; these must probably be looked for in
the incomplete radial symmetry of certain eggs—due possibly
to pressure in the uterus—and to the slight squeezings and distortions the eggs may be subjected to when they are being taken
from the Frog.
 
It will be seen that the relation between the sperm-path and
first furrow is closer than that between the latter and the sperm
entrance. This is because though the furrow may be placed to
one side of the entrance point, it may still be parallel to the path ,
or, if not to the ‘penetration ’ path then to the inner or ‘copulation ’ path, as observed by Roux. This ‘ copulation’ path is
usually observed when the penetration path is turned away from
the first furrow, that i, when it has not been directed towards
the egg-axis.
 
The same data give the position of the point or of entrance
with regard to the direction of ‘pressure ’ and ‘gravitation
symmetry’. In the (a) series the sperm tends to enter in
the direction of ‘pressure’, that is, on that side of the egg on
which it is in contact with its neighbours. Hardly a single
spermatozoon enters on that side of the egg on which the white
pole had been turned up, and very few on the opposite side.
 
It is scarcely possible to suppose that either the compression of
the egg or the gravitation plane brings the spermatozoa round to
the side of compression, but it may be imagined that either by
capillarity or by some chemotactic stimulus the spermatozoa are
especially attracted to the point where the rapidly swelling coats
of adjacent eggs come into contact, and that therefore fertilization
is principally effected upon this side. This explains why the first
furrow lies so often in this direction. The pressure may of course
afiect the position of the planes in the egg later on.
 
When the eggs are spaced the sperm enters on any side at
random.
The deviation of the sperm entrance from the egg-axis (the
angle between sperm-entrance radius and egg-axis) varies in the
two series of observations. When the eggs are spaced and the
axes vertical, the sperm enters mainly near the equator, never
near the animal pole; when the eggs are compressed and the axis
horizontal, usually at about 45° from the axis, though it may
enter near the pole or near the equator. This difierence obviously
depends on the diiference in the initial position of the eggs on
the slide. The deviation has apparently very little effect on any
of the planes we have been considering.
 
Finally, let us try and gain some conception of the mechanism
by which the direction of the furrow depends on the point of sperm
entry. It is apparently quite simple, for the sperm-path is
directed usually towards the axis, the sperm nucleus travels along
that path to meet the female nucleus, which is also in the axis,
the centrosome of the sperm divides at right angles to that path,
the fertilization spindle is developed between the diverging
centrosomes and cell-division takes place in the equator of the
spindle ; the first furrow includes therefore the sperm-path.
Should, however, the ‘penetration ’ path not be exactly radial,
for whatever reason, the sperm nucleus turns aside to meet the
female pronucleus, there is a ‘ copulation’, as distinct from a
‘ penetration’ path, the centrosome divides at right angles to
the former, and this, then, is included in or parallel to the plane
of the furrow. In those cases in which the sperm-path is parallel
to the furrow it is always quite close to it, and we may suppose
perhaps that the first division "has not been quite equal. (The
division of the centrosomes has not, I believe, been observed in
the Frog, and the foregoing description has been taken from the
Axolotl. In this genus the definitive centrosome is formed from
the sperm nucleus, when the latter has already penetrated some
little way into the egg.) .
 
The causes of the formation of the grey crescent which marks
the symmetry plane are not so clear.
 
 
Roux describes it as being due to the immigration of superficial pigment. Now we have strong reason for believing that
both the entrance-funnel——produced when the spermatozoon first
touches the egg-—and the sperm-sphere are local aggregations of
watery substance. The accumulation of what appears to be a
more watery substance about the middle piece which has been
observed in the Axolotl,appears also to occur in the Frog: at least
the same formation of large clear vacuoles in the sperm-sphere may
be seen in the latter as in the former. Should this be actually so,
we may suppose that the streaming movement centred in the
entrance-funnel and sperm-sphere is responsible for drawing away
the pigment from a certain region of the surface; hence the grey
crescent. The sperm-sphere is on the inner side of the sperm
nucleus: hence the grey crescent would appear on that side of
the egg which is opposite to the entrance of the spermatozoon,
should no disturbance of the streaming movement have taken
place, and, since the sperm-path is radial, would be symmetrically
disposed with regard to it. In this case, fertilization meridian,
sperm-path, grey crescent and plane of symmetry, first furrow,
and, later on, sagittal plane, would all coincide. There is, as
we have seen, a very fair correlation between the sperm-entrance
and the first furrow, and again between the sperm-path and the
grey crescent. But should some other streaming movement of
the cytoplasm be set up by the gravitation of the heavy yolk
particles, or by pressure, or by light, then the relation between
the two processes, the division of the centrosome which determines the direction of the first furrow, on the one hand, and
on the other, the streaming movement towards the sperm-sphere
which determines the position of the grey crescent, would be
disturbed, and while the entrance point of the sperm might still
continue to determine, though not so completely, the position of
the furrow, it might come to be without relation to the symmetry
of the egg and of the embryo; and this is what is actually
observed.
 
Though it is diflicult to assign the exact cause of each and
every deviation from the rule, this much is certain, that however
they may coincide in ‘typical’ development (I use R0ux’s
expression), the factors which determine cell-division, and those
which determine differentiation, may be influenced by different
external causes in widely diifering ways, and are therefore presumably distinct. Nor does this artificial separation of the two
processes in any wise prejudice the complete normality of the
 
development of the embryo".
 
 
Lillie has shown (Jozmz. Esp. Z002. iii. 1906) that in the egg of
C’/Iaetopterus there are granules of difierent kinds which pass, in
segmentation, into definite cells. By means of the centrifuge
some of these--the endoplasmic—-may be driven to one side of
the egg, but in whatever position these organ-forming granules
may be thus artificially placed, the cleavage has the same relation
to the egg axis (as determined by the polar bodies) as in the
normal egg. The factors of cell-division are thus separable from
those of differentiation.
 
To the cases quoted in the summary on pp. 245, 246 might be
added the various instances in which an egg may be made, by
heat or pressure or shaking, or in artificial parthenogenesis, to
segment abnormally and yet give rise to a normal larva.
 
==Appendix B==
 
ON THE PART PLAYED BY THE NUCLEUS IN DIFFERENTIATION
 
(i) BOVERI has more recently (Zellen-Studim, vi, Jena, 1907)
published a very elaborate account of the irregularities produced
by dispermy in Echinoid eggs, in which are brought forward
 
still more facts in proof of the qualitative difference of the
chromosomes.
 
As has been stated above, p. 263, dispermy is induced by
the simple expedient of adding a large quantity of sperm to the
eggs. The following types of dispermy are distinguished.
 
A. Tetracentric, i. e. each sperm centre divides.
(i) 'I‘etraster, with four spindles.
 
(ii) Double spindle, i. e. the female and one male pronucleus
lie in one spindle, the other male lies aside in its spindle.
 
B. Tricentric, one sperm centre remaining undivided.
(i) Triaster, a tripolar figure with three spindles.
 
(ii) Monaster-amphiaster, the undivided sperm centre remaining apart with one sperm nucleus.
 
C. Dicentric, neither sperm centre dividing.
(i) Amphiaster, a spindle is formed between the two centres.
 
(ii) Double monaster: the centres remain apart, one with
one male, the other with the other male and the female
pronucleus.
 
The segmentation of these eggs is as follows.
 
The tetraster divides simultaneously into four, which may
either lie in one plane if the divisions are meridional, or be tetrahedrally arranged. In the first case another meridional division
ensues, followed by an equatorial, then ‘eight micromeres are
formed, eight macromeres, and sixteen mesomeres. In the latter
case not more than three cells can share in the micromere region
and only four or six of these are produced. The triaster eggs,
having divided simultaneously into three (meridionally), subsequently show six micromeres, six macromeres, and twelve
mesomeres.
 
The segmentation of the double spindle eggs is interesting and
important. Usually the egg divides across the two spindles
312 APPENDIX B
 
into two binucleate cells, but it may divide at once into four, or
into three, one of which is binucleate. The interest lies in the
binucleate cells, for they continue to produce uni-nucleate and binucleate cells until the latter divide simultaneously into four,
and this simultaneous division may sometimes involve an irregular
distribution of the chromosomes, with fatal consequences to the
cell. Bovcri had already produced evidence of the evil effects of
an irregular distribution of the 3 n x 2 chromosomes present in
triasters and tetrasters. A more detailed account is now given.
 
Of the tripartite (triaster) ova about 8 % on an average produced Plutei. In these larvae three regions may be distinguished
in the egg by the size of the nuclei (proportional to the number
of chromosomes) and the boundaries between them may be shown
to correspond to the divisions between the three blastomeres.
The form is asymmetrical in skeleton and pigment, but Bovcri
shows that both sides are normal, as though the larva had been
compounded of two types such as occur, as individual variations,
in any culture. It is suggested therefore that the slight differences in the two sides are due to difierences in the two sperms.
 
Some of the larvae have partial defects in skeleton or pigment,
or the skeleton may be much reduced on one side, or one-third of
the cells may be pathological, i. e. disintegrate in the segmentation
cavity, while the remaining two-thirds are sound and sometimes
symmetrical. In this case it is supposed that the degenerate
cells had separated from the others at an early stage, and that
the remainder had had time to recuperate. In others two-thirds
are degenerate, one-third normal, or all three degenerate. When
the three blastomeres are isolated and allowed to develop independently, segmentation is partial, with two micromeres, two
macromeres, and four mesomeres, and often all three develop
normally up to the blastula stage. After that only one or two,
rarely all three, become Plutei, the rest giving rise to stereoblastulae or stereogastrulae, full of degenerating cells.
 
The isolated quarters of tetrasters also segment partially
and normally, but few give rise to Plutei. The whole simultaneously quadripartite eggs only rarely give rise to what may be
called a Pluteus (2 cases in 1500) ; but very degenerate larvae
are found, with masses of disintegrating cells inside, which are
assigned to one of the four blastomeres. Stereogastrulae-—with
nuclei of all the same size--are frequent.
 
As has been alread mentioned, Bovcri points out that the
probability of each cell’ of a triaster receiving a complete set of
the 71. chromosomes of the species when there are 3 n x_2 to be distributed must be greater than‘ that of each cell of tetraster
obtaining a full complement, and the probability for one isolated cell must be greater than that for the whole egg. What the
mathematical values of these probabilities are Boveri does not
know, though he makes an attempt to reckon them—not
theoretically, but by means of a mechanical apparatus; the
attempt is not quite successful. The fact, however, remains that
eight per cent. of the triasters produce normal Plutei, only -06 per
cent. of the tetrasters. This does not depend on the cells receiving
too much or too little chromatin (see p. 265), nor again on the
fact that the ratio between size of nucleus and size of cytoplasm
(see pp. 268, 269) can only be satisfied by certain definite
numbers of chromosomes, and the only explanation remaining is
that for normal development of each and every part the nucleus of
each cell must contain a complete set of the specific chrosomomes ;
from which it follows that the chromosomes are qualitatively
unlike.
 
A word may be said about the double-spindled eggs (Type
A. i). The larvae from these sometimes show abnormal regions,
and this is attributed to one or more of the binucleate cells
having divided with a tetraster and irregular distribution of
chromosomes. Of all such eggs 50 % gave rise to normal Plutei.
 
The degenerative changes undergone by the nuclei of these
larvae are of several types, to be associated again with differences
in the combinations of chromosomes.
 
(ii) Boveri’s experimental proof of the qualitative difference
of the chromosomes does not of course of itself involve a belief
in the individuality of these bodies, for if the chromatin is
concerned in inheritance, it is necessary to suppose that the
number of qualitatively distinct bodies is far greater than the
number of chromosomes, and these bodies may be differently
grouped during each successive resting stage.
 
The hypothesis of the individuality of the chromosomes, i.e. of
a constancy in the manner of grouping of these particles, rests
in the first instance on such facts as those observed by Sutton in
B2-ac/:3/stola, where in the spermatogonia the chromosomes are of
dilferent sizes, which may however be arranged in pairs, together
with an odd one or accessory chromosome. 1 In the resting stage
the accessory chromosome remains apart in a separate vesicle,
while the large chromosomes lie in separate pockets of the
nuclear membrane, the small ones, each as a separate reticulum,
in the main body of the nucleus. In the spermatocyte a number
of bivalent spiremes appear, which show the same dilferences of
sizes a the pairs of chromosomes previously, and the accessory
chromosome.
 
The accessory chromosome passes into two only of the four
spermatids and is supposed to be a sex-determinant.
 
 
Similar facts have been reported by Wilson for several Insects
(see Joum. Esp. Zool. ii, iii, 1905, 1906). '
 
Wilson finds constant size differences between pairs of chromosomes, and either an accessory odd chromosome (which passes
into only one half of the germ cells) or a pair of idio-chromosomes of unequal size (one of which goes to one half, the other to
the other half of the spermatozoa), or both the accessory and the
idio-chromosomes (giving four kinds of spermatozoa). The idiochromosomes are supposed, again, to play a part in sex-determination. Several other observers have found these accessory
chromosomes, idio-chromosomes, and pairs of chromosomes of
difierent sizes in various Insects (Boring, Journ. E211. Zool. iv.
1907 ; Stevens, ibid. ii. 1905, v. 1908; McClung, Biol. Bull. iii.
1902, ix. 1905; Montgomery, Biol. Bull., vi. 1904; Baumgartner, Biol. Bull. viii. 1904-5 ,- Zweiger, Zool. Anz. xxx. 1906;
Nowlin, Jomw. Exp. Zool. iii. 1906); in Spiders (Wallace, Biol.
Bull. viii. 1904»-5 ; Berry, Biol. Bull. xi. 1906); and in Myriapods (Blackman, Biol. Bull. v. 1903 ; Medes, Biol. Bull.
ix. 1905).
 
It is a noteworthy fact that the accessory chromosome retains
its individuality in the resting stage (looking like a chromatin
nucleolus), while the others break up. The belief in the individuality of these others rests therefore on the constancy of the relative sizes from generation to generation.
 
Further support for the hypothesis may be derived from theoretical speculations. VVe know that only 2; (one-half the normal
number) chromosomes are necessary for normal development
provided that they comprise a complete set. In sexual reproduction n maternal unite with n paternal. A study of the reducing division shows that 1: whole chromosomes first pair with
and are then separated from or whole chromosomes, and that
when they dilfer in size those of the same size pair together, and
it looks as though paternal were here separated from maternal,
though the distribution of paternal and maternal to the two cells
will difier, almost certainly, in diiferent cases.
 
If the particles of which the chromosomes are composed are
also to be paired and separated, it would appear to be necessary
that their groupin should be constant, in other words that the
chromosomes shou d retain their individuality.
 
(iii) A case of heterogeneous fertilization between eggs of Seaurchins and the sperm of Anletlon has been described above
(p. 262). Loeb has recently succeeded in rearing Plutei from
the eggs of Slrongylocmlrolue fertilized by the sperm of a
Mollusc (0/lloroaloma). Cytological details are not given (Arc/E.
Eul. Mecfi. xxvi. 1908). ‘
 
 
==Index Of Authors==
 
Agassiz: effects of fertilization in Ctenophors, 250.
 
Aristotle: theory of development, 13.
 
— the soul in function and development, 292 sqq.
 
— mechanism and teleology, 296.
 
Auerbach :' segmentation of Ascuris
nigrovenosa, 33.
 
von Baer, 16.
 
Balfour: effect of yolk on segmentation, 29, 88.
 
Bataillon: monstrosities
osmotic pressure, 120, 135.
 
—- artificial parthenogenesis, 124.
 
Bergh: cell-division in germ-bands
of Crustacea, 34.
 
Berthold: surface-tension and celldivision, 41, 42.
 
Bischofl‘, 16.
 
Blane: effect of light upon the
development of the Chick, 94, 96.
 
Boas: rate of growth in man, 63.
 
— change of variability, 73, 74.
 
— diminution of correlation coefiicient, 75.
 
Bonnet : emboitement, 14.
 
— preformation, 15.
Bonnevie : diminution of chromosomes in Ascaris lumbricoidcs, 258.
Born : gravity and development, 18,
88-85.
 
— pressure experiments on Frogs’
eggs, 34, 35.
 
Boveri : early development of Slrongylocentrotus, 23, 183-185.
 
— egg of Strongylocentrotus stretched,
39.
 
— suppression of micromeres in
Strongylocentrotus, 186.
 
-— causes of the pattern of segmentation, 197.
 
— karyokinetic plane, sperm path,
:11 ng first furrow in Strongylocentrotus,
 
8 .
 
— potentialities of? animal and vegetative cells, 192.
 
— stratification of cytoplasmic substances, 242, 280.
 
-- characters dependent on cytonlmam in Flnhinnid larvae. 261.
 
due to
 
Boveri : diminution of chromosomes
in Ascaris megalocephala, 252, 255-257.
 
— due to a difference in the cytoplasm, 257.
 
— hybrid larva from enucleate egg
fragment with characters of male
parent, 253, 258-260.
 
— irregular distribution of chromosomes a cause of abnormality, 253,
263-266.
 
— individuality of chromosomes and
chromatin, 256, 263.
 
—part played by nucleus in differentiation, 266, 285.
 
—possiblo significance of reducing
divisions, 266.
 
— number of chromosomes, size of
nucleus, and size of cell, 68, 267,
268.
 
—2méclear division not qualitative,
 
6 .
 
Bowditch: rate of growth in man,
63.
 
-- change of variability, 73.
 
Brauer : Branchipus, 22, 24.
 
Brooks: Lucifer, 22.
 
de Butfon : Preformation, 15.
 
Bullzt: artificial parthenogenesis,
12 .
 
Bumpus: change of variability in
Litlorina, 71, 72.
 
Bunge: respiration of Ascaris, 112.
 
Castle : see Davenpofl: and Castle.
 
Chabry: segmentation furrows and
embryonic axes in Ascidians, 229.
 
—- development of isolated blastemeres in Ascidians, 229, 230.
 
Child : critique of Driesch’s vitalism,
292, note.
 
Chun : isolated blastomeres of Ctenophora, 209.
 
Conklin: maturation, fertilization,
and development of Cynthia, 230236.
 
— development of isolated blastemeres in Oyntlzia, 237.
 
— development of pieces of gastrula
in Cynthia, 238.
 
— streaming movements of protonlnsm. 40.
316 INDEX OF
 
Crampton : isolated blastomeres of
Ilycmesaa, 215, 216.
 
— efieot of removal of the polar lobe,
217.
 
Dareste: mechanical agitation of the
Hen’s egg, 89.
 
— electricity, 91.
 
Davenport : catalogue of ontogenetic
processes, 4 sqq.
 
— definition of growth, 58.
 
— rate of growth, 69.
 
— the role of water in growth, 58,
59, 115, 116.
 
- and Castle : acclimatization of eggs
of Bufo to heat, 100.
 
Delage : causes of artificial parthenogenesis, 124.
 
-- number of chromosomes in artificial parthenogenesis and in merogony, 125.
De Vries : importance of potassium
for turgor of plant-cells, 146.
 
Doncaster: hybrid Echinoid larvae,
26].
 
Driesch: effect of light in development, 94.
 
— abnormal segmentation in Erhinus
produced by heat, 105.
 
— Anenteria, produced by heat,
106.
 
—- segmentation made irregular by
dilution of sea-water, 118.
 
—— pressure experiments on Echinoid
eggs, 37, 38, 185, 240.
 
—- cell-division suppressed by pressure and dilute sea-water, 55; and
by heat, 105.
 
—nuclear division not qualitative,
186.
 
— blastomeres disarranged, 187, 188.
 
— isolated blastomeres of Echinoids,
190, 191, 193, 194.
 
— potentialities of animal and vegetative cells, 193, 194, 201, 242, 243.
 
— fragments of blastulae and gastrulae in Echinoderms, 194.
 
— potentialities of ectoderm and
agghenteron, and their limitations,
1 .
 
— development of egg fragments of
Echinoids, 195, 196.
 
— germinal value, surface-area of
larvae, and number of cells, 197199, 269.
 
— one larva from two blastulae, 202.
 
— and Morgan : isolated blastomeres
of Ctenophora, 210, 211.
 
—2e1gg-fragments of Ctenophora, 30,
 
2!
 
AUTHORS
 
Drgggchz development of Myzostoma,
 
— isolated blastomeres and parts of
larvae in Phallusia, 288, 289.
 
— first furrow and sagittal plane in
Echinoids, 250.
 
— characters which depend on cytoplasm in Echinoid larvae, 261, 262.
 
— number of organ-forming substances in cytoplasm, 246, 284,
286.
 
—— theory of egg-structure, 281, 286,
292.
 
— reason for limitation of potentialities, 192-194, 201, 212, 242, 243,
281, 282, 284, 291.
 
--fate a function of position, 188,
282.
 
—- return of displaced mesenchyme
cells in Echinus, 274.
 
- stimuli in ontogeny, 20, 277, 28"284.
 
— part played by nucleus in differentiation, 266, 284, 285.
 
—— equipotential and inequipotentiul
systems, 176, 277, 285.
 
— rhythm of development, 3.
 
—- harmony of development, 284.
 
—- composition in development, 3,
285.
 
— self-difierentiation, 284.
 
—- teleology, static, 286, 291, 292,
297.
 
— —- dynamic, 291, 292, 297.
 
— vitalism, 20, 289 sqq.
 
Edwards : physiological zero for
Home egg, 102.
 
-- growth without differentiation,
104.
 
Endres and Walter : post-generation
of missing half-embryo, 171.
 
Eycleshymer: first furrow
sagittal plane in Necturus, 168.
 
and
 
Fabricius : views on development,
13.
 
Fasola : electric currents, 91.
 
Fehling : growth of the human
embryo, 59, 60, 63.
 
Feré : effect of sound-vibrations upon
the Chick, 90.
 
_ ._ of light, 96.
 
— malformations due to high temperatures, 105. .
 
—- need of oxygen for the Chick, 109.
 
—— monstrosities produced by various
chemical reagents, 18,2.
INDEX OF AUTHORS
 
Fischel, A. : hybrid Echinoid larvae,
261.
 
— variability of Duck embryos, 71.
 
Fischel, H. : isolated blastomeres of
Ctenophora, 210, 211.
 
-— derangement of blastomeres in
Ctenophora, 211.
 
Fischer: artificial parthenogenesis,
124. ’
Foot : polar rings in Allolobophom,
 
251.
 
Garbowski : function of pigment
ring in Strongylocentrotus egg, 192.
— first furrow and sagittal plane in
 
Echinoids, 260.
 
— grafting of blastulae fragments of
Echinus, 202.
 
Gerassimow: size of nucleus and
cells in Spirogyra, 269.
 
Giacomini: need of oxygen for the
Chick, efiect of low atmospheric
pressure, 109, 110.
 
Giardina : difierentiation of chromatin in female cells of Dytiscus.
 
Godlewski : the respiration of the
Frog’s eg, 110, 112, 113.
 
-— heterogeneous cross-fertilization,
262.
 
Graf : fusion of blastomeres, 56.
 
Greeley: artificial parthenogenesis
produced by cold, 108.
 
— low temperatures and absorption
of water, 108.
 
Grobben : Cetochilus, 22.
 
Groom : effect of fertilization in
Cirripedes, 250.
 
Gigiber: regeneration in Protozoa,
 
54.
 
Gurwitsch : monstrosities produced
in Amphibian embryos by chemical
reagents, 120, 123.
 
Hacker : Cyclops, 22.
 
Haeckel: recapitulation, 16.
 
— development of fragments of
blastulao of Crystallodes, 181, note.
Hr;ller : preformation and epigenesis,
 
5.
 
Harvey: epigenesis, 13.
 
— metamorphosis, 14.
 
Hecker: growth of the human embryo, 62, 63.
 
Hansen: growth of guinea-pig embryos, 62.
 
Herbst : potassium, sodium, and
lithium larvae of Echinoderms,
136-140.
 
—- significance of monsters for origin
of variatiops, 141.
 
317
 
Herbst : necessity of elements present
in sea-water for normal development of Echinoid larvae, 141 sqq.
 
—— separation of blastomeres of Seaurchins in calcium-free sea-water,
 
45.
 
— stimuli in ontogeny, 20, 272, 273,
285.
 
— formation of Arthropod blastederm oxygenotactic, 114.
 
—— arms of Plutous due to presence of
skeleton, 187, 138, 144, 149, 274, 275.
 
I-Ierl itzka, development of half-blastomeres of Newt, 173.
 
Hertwig, 0. : centrifugalized Frog’s
egg, 29, 87.
 
—- rules for nuclear and cell division,
31, 32, 85.
 
— — confirmed by pressure experiments, 34-36.
 
— gravity and Echinoderm eggs, 78.
 
—— insemination of Frog's egg, 79.
 
— cardinal temperatures for Rana
 
fusca. and csculenta, 97.
 
— monstrosities produced by high
and by low temperatures, 99.
 
— temperature and rate of development, 100.
 
—— monstrosities produced in Amphibian embryos by sodium chloride,
119, 135.
 
— first furrow and sagittal plane in
Frog's egg, 165.
 
— compressedeggs: disproof of qualitative nuclear division, 34—86, 168,
169, 240.
 
— development of half-blastomere of
Frog’s egg, 169.
 
— mutual interactions of developing parts, 271, 285.
 
Hertwig, 0. and R. : fertilization
processes altered by heat and cold,
107.
 
— — by alkaloids, 126 sqq., 263.
 
His: mechanical explanation of
development, 3.
 
—- germinal localization, 17, 158.
 
— the blastoderm oxygenoti-opic,114.
 
Hunter: artificial parthenogenesis
by concentrated sea-water, 124.
 
Iijima: spiral asters in Nephelis egg,
40.
 
Jenkinson: pressure experiments on
eggs of Antedon, 37, note.
 
— abnormalities of Frog embryos
produced by various solutions not
due to increased osmotic pressure,
120, 133-136.
318
 
Jenkinson: plane of symmetry, first
furrow and sagittal plane in Frog's
egg, 165-168.
 
Jennings: fertilization spindle in
Asplanclma, 34.
 
Kaestner: cardinal temperature
points for the Hen‘s egg, 102.
 
— malformations due to low tem~
peratures, 104. '
 
Kant : teleology, 286-289, 292, 297.
 
Kastschenko: injuries to blastoporic
lip in Elasmobranchs, 178.
 
Kathariner: gravity and the gray
crescent of the Frog's egg, 86.
 
King : cause of differentiation of lens,
276, 276.
 
Knowlton : sec Lillie and Knowlton.
 
Kolliker: 16.
 
Kopsch : first furrow and sagittal
plane in Frog's egg, 165, 168.
 
—— efl'ect of injuries to blastoporic lip,
178.
 
Korschelt: fusion of ova in Ophryotmcha, 202.
 
— nucleus of egg-cell in Dyfiscus, 252. .
 
Kostanecki and Wierzejski: efi'ect of
fertilization in Physa, 250.
 
Kowalewsky: 16.
 
Kraus : the role of water
growth of plants, 58.
 
Lang : effect of fertilization in Polyclads, 250.
 
Leibnitz : preformation, 15.
 
Lewis: causes of formation of lens
and cornea, 275, 276.
Lillie and Knowlton: eflect of low
temperatures in Amphibia, 100.
— temperature and rate of development, 101.
 
Lillie: effects of salts on ciliary
movement, 135.
 
— ghysiologically balanced solutions,
1 6.
 
in the
 
— toxicity and valency, 136.
 
Loeb : suppression of cell-division
in Echinoids and Fishes, 56, 117.
-— eflect of light in development, 94.
—the respiration of Otmolabrua and
 
Fundulua eggs, 111.
 
—— the respiration of the ova of
Echinoids, 112.
 
— function of oxygen in regeneration
of Tubular-ia head and other processes, 114, 278, 274.
 
-— efi'ect of hypertonic solutions on
Fundulus and Arbacia eggs, 117.
 
--exovates produced by dilute seawater, 118, 190, 194, 195.
 
INDEX or AUTHORS
 
Loeb: artificial parthenogenesis,
121, 124.
 
—- etfect of potassium cyanide in prolonging life of ova, 131, 132.
 
— eflect of certain salts on Fundulus
embryos and on Plutei, 135.
 
— toxicity and antitoxicity functions
of valency, 186.
 
-— effect of alkalies, 151.
 
— effect of gravity on Anmmularia,
272, 273.
 
-gégterogeneous cross-fertilization,
 
Lombardini : electric currents, 91.
 
Lyon : need of oxygen for the eggs of
Arbacia, 112.
 
— action of potassium cyanide, 132.
 
Malebranche : preformation, 15.
 
Malpighi: preformation, 14, 15.
 
Marcacci : mechanical agitation of
Hen's eggs, 90.
 
Mark: spiral asters in eggof Lz‘maac,40.
 
Mathews: artificial parthenogenesis
by mechanical agitation, 90.
 
—— effects of atropine and pilocarpine
on Echinoderm eggs, 131.
 
—toxicity and decomposition tension,
136.
 
— see also Wilson (E.B.)and Mathews.
 
Mencl : formation of lensin SaImo,276.
 
Metsclinikoif : separation of blastemeres of Oceania, 181.
 
-—fusion of blastulae in Mitrocoma, 202.
 
Minot : rate of growth defined, 60.
 
—— change of rate of growth of guineapigs, 61.
 
— - of rabbits, 62, 68.
 
— — ofchickens, 67.
 
— coeflicients of growth, 65.
 
— senescence, 65.
 
-- increase of cytoplasm, decrease of
mitotic index, 65.
 
— change of variability in guineapigs, 71. _
— genetic restriction, 246, 277.
Mitrophanow: malformations due to
low and high temperatures, 104.
— necessity of oxygen for the Chick,
109.
 
Moore : sodium sulphate an antidote
to sodium chloride, 135, 186.
 
Morgan : suppression of cell-division
in Arbacia, 56, 118.
 
- gravity and the gray crescent of
the Frog's egg, 86.
 
-— monstrosities produced by low
temperatures in Ranapaluslris, 100.
 
— need of oxygen for the Frog's egg,
110.
INDEX OF AUTHORS
 
Morgan :lithium salts used to produce
alzlgéiormalities in Frog's eggs, 120,
 
— attempts to induce
parthenogenesis, 124.
 
— number of chromosomes in artificial parthenogenesis, 125.
 
— artificial parthenogenesis produced
by cold, 108.
— first furrow, plane of symmetry,
and sagittal plane in Frog's egg,
165,168.
 
— development of half-blastomere of
 
Frpg's egg ; post-generation, 170,
 
17 .
 
— development of vegetative cells of
Frog’s egg, 173.
 
— potentialities of half-blastomeres
in Teleostei, relation of flrstfurrow
tn sagittal plane, effect of removal
of yolk, 178.
 
— effect of injuries to blastoporic lip,
179.
 
— number of cells in partial larvae
of Amphioxus, 181.
 
— potentialities of ectoderm in
Echinoids, 195.
 
— development of egg-fragments of
Echinoids, 197.
 
— number of cells in partial larvae
of Echinoids, 198.
 
— fusion of blastulae of Sphaerechinua,
201.
 
— and Driesch: isolated blastomeres
and egg-fragments of Ctenophora,
210-212.
 
— micromercs of Ctenophore egg, 30.
 
—- characters of hybrid Echinoid
larvae, 260.
 
Moscowski : gravity and the gray
crescent of the Frog's egg, 86.
 
Miihlmann : prenatal growth-rate
in man, 64.
 
artificial
 
Nfigeli : permutations of original
elements in development, 286.
 
Pander: 16.
 
Pearson : variability in man, 73.
 
Pfliiger: isotropy of the cytoplasm,
18, 158.
 
—--influence oi’ gravity on development, 18, 78, 81-83, 168.
 
-- rule for direction of nuclear
division, 32, 85.
 
Plateau : principle of least surfaces,
41, 43.
 
Platnerz 280.
 
Pott : growth of the Chick, 59, 60, 67.
 
319
 
Pott and Preyer: respiration of the
Chick, 112.
— loss of weight of Hen’s egg due to
evaporation from albumen, 115.
Preyer : rate of growth, 60.
 
Quetelet: change of rate of
in man (weight), 68.
 
— — (stature), 69.
 
— — (other dimensions), 90.
 
growth
 
Rauber : efiect of reduced atmospheric pressure on the Frog’s egg,
110.
 
— elfect of pure oxygen on the eggs
and tadpoles of the Frog, 118, 114.
 
Reichert: 16.
 
Remak : 16.
 
Robert : mechanics of spiral segmentation, 45-47.
 
— rate of growth in man, 68.
 
—-— change of variability, 73.
 
Rossi : efi‘ect of electricity on
Amphibian eggs, 91.
 
Roux : aims of experimental embryology, 13.
 
— ‘Mosaik-Theorie ’ of self-differentiation, 17, 158, 279, 286, 297.
 
— qualitative nuclear division abandoned, 19, 159, 240.
 
— idioplasm and reserve-idioplasm,
159, 266.
 
— a half-embryo from one of first
two blastomeres and post-generation of missing half, 159, 162.
 
— coincidence of first furrow and
sagittal plane in Frog's egg, 17, 159,
165. '
 
— the spermatozoon and symmetry
of the Frog's egg and embryo, 80,
165, 247, 248.
 
— meaning of karyokinesis, 252.
 
— dependent diflerentiation, 17, 158,
277, 286.
 
— functional adaptation, 290.
 
-— specific gravity of contents of
Frog’s eg, 79.
 
—- gray crescent of Frog's egg, 80, 165.
 
— influence of gravity on the Frog's
egg, 85-87.
 
— effect of electricity upon the Frog’s
egg, &c., 92.
 
— light and development, 93.
 
— segmentation of Rana esculenta, 26.
 
—- Frog's eggs compressed in small
tubes, 39, 40.
 
— comparison of systems of oil drops
and segmenting ova, 49-58.
 
— cytotropism, 55, 278.
320
 
Roux: cytotaxis, 55.
 
— cytochorismus, 45.
 
-— cytarme, 45, 53.
 
— cytolisthesis, 58.
 
— ‘ Framboisia’, 135.
 
Ruseoni : electric currents, 91.
 
Sachs : law of direction of cell
division, 28.
 
Sala: fertilization processes altered
by cold, 108.
 
- fusion of the eggs of Ascaris, 202.
 
Samassa: effect of pure oxygen at
pressures on the Frog's egg,
 
— effect of lack of oxygen on the
Frog's egg, 119.
 
— effect of various gases on the eggs
of Ascaris, 112.
 
—development of animal cells of
Frog's egg, 173.
 
— Schaper: development of tadpoles
after removal of brain and eyes,
175.
 
—- cause of differentiation of lens,
275.
 
Schulze, F. E. :
Sponges, 22.
Schulze, 0.: gray crescent of Frog’s
 
eg, 80, 247.
 
—— gravity and the Frog’s egg, 86.
 
—- effect of low temperatures on the
Frog's egg, 100.
 
—— first furrow and sagittal plane in
Frog's egg, 165.
 
— double monsters from Frog’s egg,
171.
 
Seeliger : hybrid Echinoderm larvae,
260, 269.
 
Selenka: first furrow and sagittal
plane in Echinoids, 250.
 
Semper: rate of growth in Limnaea, 67.
 
Smith: Peltogaster, 24.
 
Sollmann : after effects of hypertonic
solutions, 124.
 
Spemann : development ofconstricted
Newt's eggs, and embryos, 174, 175.
 
— causes of formation of lens and
cornea, 275, 276.
 
Sumner: injuries to blastoporic lip
of Teleostei, 178, 246.
 
Sutton {individuality of chromosomes
in Brachyslola, 256.
 
Swammerdam : preformation, 14, 15.
 
segmentation of
 
Vejdovsky : unequal centrosomes in
dividing pole-cells, 31.
 
— polar rings in Rhym.-hclmis, 251.
 
Vernon: rate of growth in Strongmlocmtrotus, 67, 70.
 
INDEX or AUTHORS
 
Vernon : alteration of variability in
Echinoid larvae, 71, 74.
 
-— effect of light on Echinoid larvae,
95, 96. '
 
— effects of change of temperature
on Echinoid larvae, 106, 107.
 
-— change of variability produced
by heat, 107.
 
— and by chemical agency, 141, 156.
 
—poisonousness of carbon dioxide
to Sea-urchin eggs, 112.
 
— characters of hybrid Echinoid
larvae, 261.
 
Verworn : behaviour of Protozoa in
an electric current, 93.
 
— regeneration in Protozoa, 254,
note.
 
Walter, sec Endres and Walter.
 
Weber : law of stimuli, 272.
 
Weismann: qualitative
division, 19, 297.
 
— idioplasm, and reserve—idioplasm,
159.
 
Weldon : growth-rate in Carcinus, 71.
 
— change of variability in Carcinus,
72.
 
— — in Clausilia, 73.
 
Wetzel : double monsters
Frog’s egg, 172, 245.
 
Whitman : polar rings in Clepsine,
251.
 
Wierzejski, see
Wierzejski, 250.
 
Wilson, 0. B. : malformations of
Amphibian embryos, 120.
 
— acclimatizution to salt-solution,
136.
 
Wilson, E. B. :
phioxus, 26.
 
—— segmentation of Renilla, 55, note.
 
— unequal centrosomes in dividing
pole-cells, 31.
 
—pressure experiments on eggs of
Nareis, 39, 213, 240.
 
- cytology of artificial parthenogenesis, 124.
 
— development of isolated blastemeres in Amphioxus, 179, 180.
 
—— isolated blastomeres of Oerebratulus,
and fragments of blastulae, 205,
206.
 
— isolated blastomeres of Patella,
218-222.
 
—- of Dentalium, 225, 226.
 
—— removal of polar lobe, 224.
 
— effect of fertilization, 222, 223.
 
— development of egg-fragments,
226, 227.
 
nuclear
 
from
 
Kostanecki and segmentation of Am
 
Wilson (E. B.) and Mathews : spermpath, egg axis, fix-st furrow, and
embryonic axes of Toacopneustes,
185, 249, 250. ‘
 
Windle: effect of magnetism and
electricity on development, 91.
 
Wolff : epigenesis, 16. '
 
Yatgu: egg-fragments of Cerebratulus,
 
2 7.
 
Yung: effect of light on tadpoles,
etc., 94.
 
Zeleny : egg-fragments of Cerebratulus,
206, 207.
 
Zelinka : fertilization
Callidma, 34.
 
spindle in
 
Jnxntsonr’ Y
 
Ziegler : heterodynamic centrosomes, 80.
 
.— formation of micromeres in Cteno
phora, 209, note.
 
-— pressure experiments on egg
gaéiinoids and Ctenophora,
 
— fertilization of Diplogaster, 84.
 
— egg and embryonic axes, 250.
 
Zoja : isolated blastomeres of Hydromedusae, 181, 182.
 
—— animal and vegetative cells of
Strongylocentrotus, 198.
 
Zur Strassen : segmentation of
Asoaiis, 81.


— fusion of the eggs of Ascaris.
Index of Authors


s of
Addenda
88,


==Addenda Et Corrigenda==
==Addenda Et Corrigenda==

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Jenkinson JW. Experimental Embryology. (1909) Claredon Press, Oxford.

Jenkinson (1909): 1 Introductory | 2 Cell-Division and Growth | 3 External Factors | 4 Internal Factors | 5 Driesch’s Theories - General Conclusions | 6 Appendices
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Experimental Embryology

Experimental Embryoijogy

By

J. W. Jenkinson. M.A.. D.Sc.

Lecturer in Embryology in the University of Oxford

(1909)

Preface

For the biologist there are, I conceive, in the main two problems. One is to give an account of those activities or functions by means of which an organism maintains its specific form in an environment. The other is to find the causes which determine the production of that form, whether in the race or in the individual. The solution of the first of these problems is the business of physiology, in the usual sense of the term. The second falls to morphology.


It is with the origin of form that we are here concerned, and in particular with its origin in the individual. The endeavour to discover by experiment the causes of this process — as distinct from the mere description of the process - is a comparatively new branch of biological science, for Experimental Embryology, or, as some prefer to call it, the Mechanics of Development (Entwicklungsmechanik), or the Physiology of Development, really dates from Roux's production of a half-embryo from a. half-blaatomere, and the consequent formulation of the ‘ Mosaik-Theorie’ of self-differentiation. That hypothesis has been the focus of much fruitful criticism and controversy, the experiment has been followed by many others of the same kind, and the present volume is an attempt to sketch the progress of these researches and speculations on the nature and essence of differentiation, as well as of those which deal with growth, cell-division, and the external conditions of development.


In writing this review I have had the very great advantage of an excellent model in the textbook of Korsehelt and Heider (Lehrbuch cler fucrgleichemleat Entwio/cluugsgeschiclzte (Zer 1ve'rbelZo.~e'n T/u'c7'e, Allgemeiner Theil, Jena, 1902). I have indeed followed the general arrangement adopted by these authors fairly closely except in one respect. I believe so strongly that the processes of growth and cell-division, though they always (in the Metazoa) accompany, are yet distinct from, differentiation, that I have felt justified in treating them in a chapter apart from the other internal factors of development. The external factors—whether of growth, celhdivision, or differentiation - are discussed in Chapter III, and the ground is thus cleared for a consideration of the real problem — the differentiation of specific form.

The last chapter is devoted to the theories, scientific and philosophical, of Hans Driesch. I sincerely hope that Herr Driesch will allow my great admiration for the former to atone in some measure for my inability to accept the tenets of nee-vitalism.


It is a very great pleasure to me to acknowledge my indebtedness to the Delegates and Secretaries of the Clarendon Press, and in particular to Professor Osler, for undertaking the publication of this book, as well as for the pains which have been expended in its preparation. Dr. Osler also took the trouble to read through the whole of the manuscript, and Mr. G. V. Smith and Dr. Haldane have been kind enough to look through certain chapters.


To Dr. Ramsden I am under great obligations for his assistance in that part of Chapter II, Section 1, in which surface-tensions are discussed; to Dr. Vernon for calling my attention to Roberts’s work on Anthropometry, and to Mr. Grosvenor for the information embodied in the foot-note on p. 89. Mr. A. D. Lindsay has given me invaluable assistance in those sections of Chapter V which deal with the philosophy of Kant, while, for Aristotle, I was fortunately able to attend Professor Bywater’s lectures on the De Anima.

I can hardly express the debt I owe to Mr. J. A. Smith for much friendly counsel and criticism, although he is, of course, in no way responsible for the philosophical speculations in which I have ventured to indulge.


The illustrations are largely borrowed from Korschelt and Heider’s work, and I must thank Herr Gustav Fischer, of Jena, for his readiness in supplying the blocks. Others are from the original publications‘, and I am obliged to the proprietors for permission to make use of them. A few are my own.


In the appendices will he found an account of some recent work on the relation between the symmetry of the egg and that of the embryo in the Frog, and on the part played by the nucleus in ditt'c1-entiation.

Proceedings of the Boston Society of Natural History, the Journal of Experimental Zoology (Williams 8; Wilkins, Baltimore), the Anm'ir(rn Journal of I‘hysz'ulo_'/_I/ (Ginn & C0., Boston), ZeIIrn~Sfu(Iim (Fischer, Jena), l’erhamIlmI_r/en 410;" A/mlumis-1-hm G(‘.s'¢'”N(‘7I((fl (Fischer, Jena), Er;/cbnisse fiber din Ii'on.m'tzm'ou dcr cIu'onmta'scIzm Kernsubslmz: (Fischer, Jena), .[r¢-kin fiir mik)'osk0])i.s¢*7¢1: .»lm¢tomi(' (Cohen, Bonn), Archizv ff/"r Entwiclcluuysnwvlzanik (Engelinunn, Leipzig), and the Popular Science .llontM3/ (Appleton & Co., New York).

Contents

Chapter I Introductory

Chapter II Cell-Division And Growth

  1. Ce1l-division
  2. Growth

Chapter III External Factors

  1. Grravitation
  2. Mechanical agitation
  3. Electricity and magnetism
  4. Light
  5. Heat
  6. Atmospheric pressure. The respiration of the embryo.
  7. Osmotic pressure. The role of water in growth
  8. The chemical composition of the medium
  9. Summary

Chapter IV Internal Factors

(1) The initial structure of the germ as a cause of differentiation.

  1. The modern form of the preformationist doctrine
  2. Amphibia
  3. Pisces
  4. Amphioxus
  5. Coe-lenterata
  6. Ecliinodcrmata
  7. Nemertinen
  8. Ctenophora
  9. Chaetopoda and Mollusca
  10. Ascidia
  11. General considerations and conclusions
  12. The part played by the spermatozoon in the determination of egg-strucure
  13. The part played by the nucleus in differentiation

(2) The actions of the parts of the developing organism on one another

Chapter V Driesch’s Theories Of Development - General Reflections And Conclusions

Appendices

APPENDIX A On the symmetry of the egg, the symmetry of segmentation, and the symmetry of the embryo in the Frog


APPENDIX B

On the part played by the nucleus in differentiation

Index of Authors

Addenda

Addenda Et Corrigenda

P. 5, 5 lines from bottom, for unicellular read multicellular. P. 28, line 10, after irregular, insert and in Triclads.

P. 57. To Literature acid J. Sacns. Die Anordnung den-Zellen in jiingsten Pflanzentheilen, Arb. Bot. Inst. Wurzburg, ii, 1882. _

P. 114. To Literature add G. BUNGE. Weitere Untersuchungen iiber die Athmung der Wiirmer, Zeitsc-hr. physiol. Chem. xiv, 1890.

P. 140, line 22, for prospective potentialities read prospective significanoes.

P. 225, 2 lines from bottom, for is now placed in road has now moved into.

P. 271. To Literature add W. S. Surrox. On the morphology of the chromosome group in Brachyslola magna, Biol. Bull. iv, 1902.

P. 278. To Literature add J. W. Jnxxmsox. On the effect of certain solutions upon the development of the Frog's egg, Arch. Ent. Mech. xxi, 1906.


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